I've been tidying up the instrument panel on the Wonderbus. The 71 model year, unlike the previous model year has a brake lamp on the instrument panel (in the US, anyway). I don't have a 71 Owner's Manual, only the
1970 manual. I presume that this lamp is meant to warn of low brake fluid or pressure or something.This is a three-wire device. The wiring diagram does not clearly show all three terminals. It shows a Brown wire, which is chassis, which connects to the terminal on the bulb holder itself. There are two other terminals on the body of the device, labeled "15" and "K" (had to put on my Dr. Frankenstein magnifying goggles to read them).
I presume that the black wire (switched 12V) is meant to be connected to Terminal 15. And the red wire, which goes to items "F" on the wiring diagram ("Brake light switch with warning switch") would go to Terminal K.
This wiring, and the construction of the lampholder, indicates that you should be able to test the bulb by pushing in the lamp lens. Right now I'm not getting any lighting of the bulb, yet it is a good bulb.
I guess I'm asking for someone to confirm that my understanding of the wiring and operation is correct. Then I can poke, probe, and threaten until this potentially useful warning lamp becomes functional.