Okay, this is about my wife's '93 Volvo -- she has the "later" version, with the four fuses on the positive battery terminal (unlike earlier versions, without the fuse array), in case it makes a difference.

Anyway, I decided to replace her rear window (a.k.a., center-mounted) OEM, incandescent brake light with a new LED type. When I hooked it up, as I expected, the bulb-out warning appeared -- no surprise, I'll have to add a resistor to match the increased current draw expected by the reed switches. But that's *not* what I want to discuss here.

What I am interested in is this:

When the car is in Park or Neutral, the warning light only comes on when the brake pedal is stepped on, and goes out when I take my foot off the pedal -- just what I expected. But here is the surprising thing:
When I put the car is drive, the light comes on immediately and stays on (until I do back to Neutral)! That's regardless of whether I'm stepping on the brake pedal or not!

Did anyone know this? Can anyone explain this? I looked at my wiring diagram book (the green Volvo manual with all its pages of colored diagrams), and there is no apparent link between the shifter or transmission and the BF module. So how does the module know when the car is in-gear, or not, in order to know to keep the warning idiot light on?

Any explanation will be appreciated -- my curiosity is driving me crazy about this.

Thanks to all, and happy holidays!