Internal pix of fan switch. At 180K mine was intermit. You can open up the front of the dash and remove some small plastic strips that allow you to squirt in contact cleaner (and maybe even some emery cloth on a thin screw driver) this worked for a few months for me, but then back to wiggling, AC cutting off, etc. Pulled out dash last night (way more screws than you think are there). Switch pops out the back, held by a tab on dash plastic that must be depressed toward floor.

When I dismantled it here is what I found, a soft copper strip switch, overheated, and pitted. The pitting is from arcing on/off in same spot. The small rectangle with three bumps is moved by the selector, it has a small spring behind it to maintain contact. This pitting and mechanical wear simply make the switch contacts too thin to maintain contact. I cleaned it up with a brush and emery cloth and re-installed, but I then only had speed 1 and 2-3-4 would not wiggle on. Just too much metal worn away.

So off to the dealership for parts. My advice for others, if a squirt of contact cleaner doesn't help, plan on a new switch before pulling dash.

Update from dealer, switch not sold by itself, you have to buy the AC **** module as an assembly. Googled the Siemens number on the switch and no hits.

Took switch apart again in a bowl. Springs and jump, one for contact, one for detent. Found under closer inspection that speed four contact edge was worn down at a bevel. Stretched contact spring (it pushes between plastic slide and metal three point contact) slightly to increase tension. Got all four speeds back, but # 4 can still be wiggled and drop out. So now a trip to the salvage yard where the partz vulturez fly low and fast on Saturdays.

And of course "discovered" that previous owner had trashed the dash plastic installing after market radio, so I'll be at the junk yard looking for that, or trying to JB Welds tabs back on, or conspiring with a dismantler..... The one bright spot was that I could finally fix my aftermarket FM only radio, the PO had plugged into the wrong antena lead (one is AM/FM, the other FM only).

 

Emery cloth will do wonders to that old copper.
Unless the cooper is broken the switch is rebuildable.

 

Emery cloth will do wonders to that old copper.
Unless the cooper is broken the switch is rebuildable.

 

Contact cleaner will clean the copper but not protect it long term. I'm into vintage audio gear and I've learned a lot about those cleaners. If you purchased fader/slide cleaner and protector like those produced by Caig Laboratories, Inc, you could extend the time between cleanings. The fader/slide cleaner will clean the crap and the protector will retard the re-oxidation process.