bulb out sensor

thanks for the advice, turns out it was the bulb out sensor

If you switch bulbs from side to side, make sure the sockets are the same, or keep the sockets in their proper locations - I nearly melted my taillights when I swapped sockets on our '90 744 when I tried to find a bad bulb. They were different colors and had different connections to the tail light circuit board.

If you have a bad bulb, replace both sides and keep the old good bulb in the car as a spare.

I've had considerable trouble with my taillights.I think if the switch,relay,or sensor was bad,both lights would be out. When my bulb failure relay (the round orange one) was out, I had no brake lights at all. So I think the problem is the passenger side itself.Make sure you have good connections to the sockets,and sometimes the socket isnt in its place correctly, or the wire has come off the metal tab. Try turning it a little while someone steps on the brake pedal and see if that helps. Once the plastic part of my light melted and warped,causing it to not seat the socket in there nicely,and it didn't make a good connection. Or it could be just a bulb! Try taking the socket and bulb out of the other side (that you're sure is working,) switch places with the non-working bulb. It's aggravating to have to do this,I know. You'll get there eventually.Or maybe not. On my wagon I just gave up and ran a piece of wire from the offending taillight to the middle light and jumped it. No more trouble.