I noticed that my 993 headlight reflectors and the inside of the lenses were covered w/a dust/dirt film and were not as bright as a friend's 993 lights. I guess it's from 5+ years & 34k miles of use. The headlight assemblies are NOT airtight (open venting under the spring lock area), nor is the headlight fender bucket. 

I decided to clean the INSIDE of my headlights which turned out to be easy, fast and effective. Obviously, remove the headlights following the simple owner's manual instructions. 

The assemblies are easily opened by prying up each of the 5 catches connecting the lens assembly from the body. Two of the catches are "sealed" by a melted down plastic area. Gentle prying broke open the seals. I then gently pried the 2 parts open by working around the assembly with a broad screwdriver/putty knife. Once opened I carefully cleaned the inside lens glass & the projector glass lens with a lint free cloth & glass cleaner. I also cleaned the reflectors with a dry, clean, lint free cloth. Reassembly was easy. Just slide the 2 pieces together until the catches all snap into place. 

Result - The difference was amazing. Both reflectors & lenses now look like new (except for some external stone chips) with obvious benefits to light output. 

Warning!  Update 4/03 Contributed by Coleman

The reflective coating on the inside of the lamp is EXTREMELY fragile with  age. It's almost better not to touch the surface. 
Also if the clear plastic inserts are yellow, take 'em out and it looks  much better.