I thought adding this information to the p-car.com web site would be beneficial to everyone who may need to program additional remotes in the future.

The information is straight out of the 993 shop manual, the information starts in 1994 model year and with provisions for 95 and up and the RS model.

Note
As a result of tolerances in the ignition lock, it is not possible to define precisely the position at which the buzzer contact is closed by the ignition key.
Depending on the tolerances, the buzzer contact(S) in the ignition lock

- May be closed between position 0 and I (Fig A)

          or

- between position I and II (Fig. B)

I = terminal X (windshield wiper on)
II = terminal 15 (ignition on)
III = terminal 50 (starter)

Note
you can find the same x,15,50 symbols in the schematic section of the shop manual, which is helpful in understanding which wire is hot under what condition.

When the key is remove from the ignition lock, the buzzer contact is opened (switched off).  This signal is required for tuning the remote control units.

The position where the buzzer contact is operated can be located as follows.

Manual transmission vehicles:

The buzzer contact signal is transmitted via pin 12 in the 14-pin connector (passenger compartment wire harness, connector X4/1 under the left seat).

1.  Remove the left seat.  Do not unplug the connector; carefully remove the connector cover and connect a test lamp to pin 12 (red/black) and ground.  (What the German guys trying to say here is to connect a test light's alligator clip to a ground of the car and poke the sharp end at pin 12 wire)

2. Turn the key slowly from position 0 to positive II in the ignition lock and observe the position at which the test lamp lights up.

Tiptronic vehicles:

The buzzer contact signal can be observed on the LEDs of the selector indicator (speedometer instrument cluster).

1.  Turn the key slowly from position 0 to position II in the ignition lock.

2. Observe the position at which one of the LEDs of the selector indicator lights up.

Important!

Manual and Tiptronic vehicles:

Depending on the position of the buzzer contact, for tuning "programming the remote", the ignition key must be turned to 

- position I (windshield wipers on)

or

- the buzzer contact position (S) (See Fig. A and B above)

Fig A: buzzer contact(S) between position 0 and I.  For tuning, the key must be turned to position I (windshield wipers on).

Fig B: buzzer contact(S) between position I and II.  In this case, the key must be turned to the contact position (test lamp or selector indicator) for tuning.

Exception!

If the buzzer contact is reached just before or directly at "ignition on":
...... tuning is not possible.  In this case contact Porsche, Dept.  VKG  (Don't ask me what this means, maybe someone can tell us)

Tiptronic vehicles:

Before tuning the remote control units, check the key position at which the buzzer contact is operated.  Then tune the units. 

Tuning the remote control units

The diagram shows the sequence of events. The time required is at least 43.5 minutes.

1.  Unlock the vehicle using the remote control unit and open the door.  Do not insert the key into the ignition local (the door of the vehicle may be open or closed.)

2.  Wait 3 minutes.  After this time, the immobilizer will prime itself.  In order to ensure that you comply with times given below, please use a stopwatch or alarm clock.  If necessary, write the times down.

Note:
When the instructions below require "switch ignition on and off", it is sufficient to turn the ignition key until the ignition is witched off.

3.  Now start the stopwatch and at the same time switch the ignition on.
Leave the ignition key turned to "ignition on".
Switching the ignition on starts a sequence of operations.  After 41.5 minutes (± 1 Min.), the system reaches the status "tuning of remote-control units."

4.  After 22.5 minutes (±2 Min.), the ignition must be switched off and on again.

5.  After a total time of 32.5 minutes (±2 min.), the ignition must again be switched off and on.
Then lift the windshield wipers off the windshield (to detect ignition key position I) 

6.  After a total time of 41.5 minutes (±1 Min.), perform the following sequence, starting with the ignition on:
-set wiper switch to stage 1
-ignition off
-ignition on
-ignition off
-remove key from ignition lock
-insert key into ignition lock and turn to pos. I or to buzzer contact (not "ignition on")
If the key is in the correct position, either the wipers much be switched on in pos. I or, in buzzer contact position, the test lamp or selector LED must light up.
-Remove key from ignition lock.

7.  After the key has been removed, you have a total time of 1 minute for tuning the remote control units.  During this 1 minute, you must tune one remote control after the other to the vehicle by pressing the pushbutton.  When tuning the units, you must point the unit with the button pressed towards the control unit for at least 5 seconds.

8.  The status "tuning of remote control units" is ended when:
-4 remote control units have been tuned
or
-a 5th remove control unit is operated within the minute
or
-one unit is operated twice
or
-the minute has elapsed.

Note:
Any remote control units already assigned to a vehicle which are not tuned during a tuning session lose their validity for the vehicle.

Note:
The key should only be used for emergency unlocking in the event of a failure of the remote control unit.

1.  Hold the key in the unlocking position and open the door at the same time using the door handle.  The alarm is triggered immediately.

2.  It is not possible to deactuate the immobilizer or the alarm system.  The engine cannot be started.

3.  The door is locked again as soon as it is closed.  The central locking system remains in operation.

Note:
The key should only be used for emergency unlocking in the event of a failure of the remote control unit.
De-activation of immobilizer (with vehicle locked)

Note:
It is possible to switch off the acoustic alarm for 2 minutes so that the immobilizer may be de-activated by entering a four-digit code number specific to the vehicle.  The four digital code number (key card) is entered using the ignition key by a sequence of ignition on and off operations.  The time available for entering the number is determined by the number of ignition OFF-ON operations, starting with the left digital.  To enter "0", the ignition must be switched from off to on 10 times.

You must wait for the warning light to go on before entering the next digit.

1.  The door lock must be unlocked, locked and unlocked again in 5 sec.  The acoustic alarm is then interrupted after 1 second.

2.  Open the door within 10 seconds and switch the ignition on.  The warning light in the clock will then light up and go out again after 15 seconds.

3.  Switch the ignition off and on again within 5 seconds.  The warning light will light up and start to flash after 15 seconds.

4.  You must start the enter the code number within 5 seconds.

5.  Example: code number 1312

1 - ignition off-on
wait for warning light
3 = ignition off-on, off-on, off-on
wait for warning light
1 = ignition off-on
wait for warning light
2 = ignition off-on, off-on

6.  If the number is correct, the warning light will flash.  This is also the point at which tuning of the remote units can start.

Note:
After the warning light has flashed and the ignition has been switched on again, the engine can be started.

The immobilizer is deactivated and the alarm system and central locking system are unlocked.  If the warning light does not flash after the code number has been entered, steps 1-5 must be repeated.
Emergency deactivation does not deactivate the self-priming function of the immobilizer system.

Turning the remote control units:

Note:
A maximum of four remote control units may be assigned to a vehicle as the same time.  Additional remote control units must always be tuned to a vehicle together with the two units already assigned to it.  Before the code number is entered, the immobilizer must be primed but the alarm and the central locking systems must not be activated.  This status is reached 90 seconds after the ignition key has been removed or 3 minutes after the vehicle has been unlocked using the remote control unit if the ignition key is not turning in the lock.
 

1.  Switch the ignition on.  The warning light in the clock will go on and off again after 15 seconds.

2.  Follow the instructions in points 3 to 6.

3.  If you have reached the tuning status (point 6 warning light is flashing) the remote control units must be tuned within one minute.

4.  Press the pushbuttons of the remote control units until the LEDs of the alarm system in the doors flash (as an acknowledgment).  Operate all the remote control units one after the other.

5.  If no more remote control units are operated within 1 minute or the ignition is switched off, tuning is stopped. 

Note:
As Carrera RS vehicles are not equipped with a central locking system, both doors must be locked and unlock using the key.

To deactivate the immobilizer, the remote control unit must be operated twice in the following situation.  The immobilizer has self-primed and the ignition has not been switched on.

If the ignition is switched on before operating the remote control unit or the remote control unit is used to activate the immobilizer, the remote control unit must only be operated once.

The procedure for tuning remote control units is the same as for the 911 Carrera (993) from model '95.