The impact screwdriver is a must-have in almost anyone's toolbox. But I'm of the opinion that it's not the right tool for this particular job. Both the rotation (torque) and the impact ultimately get passed to plastic housing pieces. The impact is to the lens assembly that's a few $hundred alone. The torque is to the ear of the lens assembly until the screw head breaks loose, and it's full time on the plastic insert. The normal weapons like the impact driver and heat are wrong for this situation unfortunately. The housing is PE, reasonably resistant to low-flash solvents like acetone in the acetone/ATF brew. It might be enough, --IF-- it can find its way into the threaded insert by capillary action.

Last caution is to make darn sure you are turning the housing screws and not the adjusters. The adjusting screws thread into little plastic arms and links that guide the reflectors inside the housing. Those plastic arms and links are crazy fragile, and winding them too far in either direction almost guarantees breakage.

 

I've gone through the rounded heads/try to drill out/melt plastic/threads spin/buy replacement lamps thing.

After that the impact driver idea worked perfectly on several occasions with another 928 I owned and a couple owned by friends.

But caveat is the force of hammer will probably break filament in bulbs, but cost trivial compared with new lamp replacements (one of parts where Porsche jacked up price quite a bit I believe)

 

The fact that the housings are (now even more) fragile plastic makes this a royal bitch. If you lop the screw heads off with a hacksaw or dremel to drill them out, you can't put too much pressure. You can't lay them on a bench to drill them out, you gotta hold the housing in your hand. I screwed up my left index finger pretty bad when the drill slipped and went through my flesh to the bone. Couldn't move it for a week but no infection luckily. A true masochists dream these cars are.

 

Something to try because plastic tends to be heat/solvent sensitive: Chill down the screw with a piece of dry ice to contract the screw and cause it to break loose from the insert. HOWEVER, DON'T TOUCH THE SCREW TILL IT COMES BACK UP TO AMBIENT TEMP! Metal and plastic gets very brittle at -85* f.