1990 740 base

It's been happening for a while now, but once in a while, my "lamp out" light will go on when I'm braking, or sometimes when I'm signalling a right. I read once that the light-check system is weird in that sometimes not having matching bulbs could cause it. So of course, I try and replace all my rear lights, completely, (except the third brake light up top) and then lo and behold, it's STILL flashing sporadically. I think something might be a little messed up with the circuitry, because this NEVER happens when I have my headlamps on. If the lamp-out light goes on, I just turn on my headlights, and boom, the lamp-out light goes out. Does anyone have any idea?

Note: this car was in a flood submerged almost up to the radio deck at one point. Everything works fine, even the seat heaters. It's just that annoying lamp-out light.


Thanks.