[porwolf]

I came across a rennlist thread dealing with an add-on PWM circuit that a member there built and is selling for 928 cars. Looks very good and it uses the original Porsche rheostat to control the dimming. That should work also for 911's. I cannot think of any reason why the 911 instrument lighting is much different:

PWM dimmer for instrument backlighting after upgrading to LEDs - Rennlist Discussion Forums

And here is where you can order the dimmer as a complete unit or parts kit for do your own:

http://www.protizmo.com/Product-Info/AILD-1/