Advice on fixing stop lights 740 GL

The most likely culprit at this point is the bulb failure sensor, it's a round can plugged into the relay tray. If you pop it open you'll see two circuit boards stacked, and probably some cracked soldering. Heat up the solder joints and flow a bit of new rosin core solder into any that look suspect and you should be good to go. If you or someone you doesn't know how to solder, you can replace the sensor but you should at least look inside to verify the problem.

I was going to suggest the same thing James. But, I'll add that the red "can" relay is a pricey item new. There are often relay sets on eBay for a fraction of the cost of new.

RS

Yeah they are pricey! I would definitly suggest trying to fix the existing one first, or even take it to a TV repair shop, any tech at one of those places could fix it up in a few minutes. Pretty clever design really, the old ones use two coils wound around a reed switch and wired out of phase. So long as the current is equal, the magnetic field cancells out but any imballance results in enough magnetism to trigger the reed switch and turn on the bulb failure warning. Of course any bad connection in the whole mess will kill the lights and the sensor is very sensitive to corroded sockets and such.

I've only ever seen the relay make all 3 bulbs fail, not to say that it can't fail to allow only the 3rd light to work. I'd first look at the condition of the bulb holders, circuit cards and the condition of the plastic of the taillight itself where the bulb holders sit as it often melts there.

Didn't he say he already checked that?

Both times I've resoldered a bulb failure sensor the center brakelight still worked fine but the main brakelights and in one case, one of the taillights were out.

Yeah, but often when there's a problem with melting it's not obvious until you're looking for it.