A bay window bus has very simple air controls. Three levers, two red and one blue. The blue lever controls fresh air in from the front grill, that gets directed up into a windshield defroster (which seems weird, but makes sense from an air pressure perspective) and out of the adjustable vents. The two red levers control heat: one lever controls the volume of hot air from the heat exchangers, and the other controls whether it comes out of the kickpanel vents or out of the secondary window defrosters.
Now, the speed of the air coming out of the vents is dependent on the engine speed, as it uses air from the cooling fan, attached to the rear of the crankshaft, as a source:
To solve the little problem of “literally no heat at idle,” VW fit a booster fan that helps to blow through the heat exchangers at low engine speeds
This fan is activated by a switch on the heat volume lever (the rightmost red lever) that grounds out a wire when the lever is in the lowest position. Shown here (but with two switches, that later optioned buses had for…something):
The heater cables that went to my heater boxes were broken; the cable ends had rusted away. I made a clever “fix” to this by bending the remaining ends of the heater cables into a hook, and using a bit of coat hanger wire to bridge the gap to the heater box flap levers. While this worked, and I can open the heater box flaps fully, the lever itself on the dashboard never goes all the way down to the bottom of its travel, so it never activates the switch for the booster fan.
So, I did exactly the opposite of what somebody on TheSamba would do, and instead of buying new heater cables for $60 apiece (x2) and spending a week routing them under a rusty bus, I bought this switch, and installed it in the dashboard between the dome light switch and the hazard light switch. In the photo below, it would be sideways where the switch is that is missing the insert:
The downside is it switches a ground wire, so I can’t wire up the switch to light up when on. Oh well, it’s still labeled.
Also, if you want to have a good head scratch, google “bay window bus dashboard” and look at the switch positions. No two of them are the same. It’s like they didn’t give a shit at the factory.