Takes 15 minutes max if you are leisurely about it and is very easy. You need a Philips screwdriver, a stubby flat-bladed screwdriver and 8mm, 10mm, 13mm and 22mm sockets or spanners.
The parts needed are from the 944, PET illustration 803-05 Model 9442 Year 89:
P477.845.333 Screw (£20.96)
It's ugly, it spoils the clean look of the rear of the car, it's got to go.
Lift the plastic cap on the rear wiper arm, remove the nut (13mm), lift off the wiper arm (it remains partially attached), lift off the plastic cover, remove the next nut (22mm) then remove the rear wiper.
Move pass seat full forward, fold it forward, fold down the rear seats. Take out the 4 screws in the corners of the rear panel. Pull the rear panel forward and turn it downwards.
Unplug the connector from the rear wiper motor assembly and remove the 2 10mm bolts that face you.
The whole wiper assembly now just lifts off the bracket, which is itself removed by taking out two more bolts (8mm). Transfer the large U-shaped plastic piece to the slot in the rear panel.
Now from outside the car put one of the 2 grommets on the screw and put it in the hole in the screen. From inside the car put the O-ring into the hole over the screw, then the other grommet, then the washer, then put the nut on. The screw has a screwdriver slot in the end to hold it while you tighten the nut.
Tape down any bits of wire that look like they might rattle about and tape down the connector that used to go to the rear wiper. Put the rear panel back in place and re-fit the 4 screws.
Revel in the new, cleaner look of your rear window, not to mention the enormous 5lb weight saving.