It's the gadget with the knobs and sliders. The trim has to come off, it's no big deal. In my case the fan speed switch was falling apart inside. These swap between late 944 and 968 however the 968 has a detent pot for the temp. I'm running a 944 head in my 92 with no issues.
Removal instructions and pics are about halfway down this page:
http://www.clarks-garage.com/shop-manual/body-24.htm
The much earlier versions of this device (85.5, 86, ?) is probably compatible but lower tech inside so if you get a part to swap in I'd suggest at least 88 and newer.
If it's the ballast resistor assembly these can be cleaned up with a little acid on the overtemp thermal contactor. I had that problem in an S2. You can open up the contacts with a toothpick and get a drop or two of acid in there, or I have heard of people soaking the contacts in vinegar somehow. I'd use acid if you have an old battery around, or a strong acidic cleaner like CLR or kaboom. A drop of battery acid on a toothpick will work well if you are comfortable with that sort of thing. If you have the assembly in hand you can see what you are trying to fix. Clear out any acid from the switch with with water or diluted alcohol and dry it well before reinstalling it.
