Vintage Cars: 1978 MGB electrical problem, light sockets, backup lights

Question
QUESTION: My left brake light does not work.  When I turn my lights on, only one filament seems to be working.  I have changed bulbs but this does not fix the problem.  When I put the car in reverse, the back up lights go out and only the right brake light works.  Where should I start to look for the problem.
  Thank You for your help.

ANSWER: Hi Jim--

Connections, connections, connections. Suspect:

1) In-line connections like spade or bullet. It's not the spades or bullets but the unreliable crimps. Solder at the crimp is best.

2) Dirty terminals at the brake pressure switch or at the rear light light sockets, causing a damp leak to ground. Squirt with WD40 and wipe down.

3) Bad socket or pressure-switch *ground*. Loosen light sockets or switch and grind against the sheet metal to make a good ground. Then retighten.

In your particular case (a confusing mixup of backup and brake lights) it's possibly a ground problem somewhere in the trunk sockets, leading to cross-connection of the backup and brake connections.

I think it extremely unlikely that anything inside a wiring harness has gone bad, but if the harness line to the rear lights has been broken into in the past, suspect the new connection.

Final advice: I doubt it's useful to do this logically from the weird symptoms: just cleaning everything should be quicker! Hope it works.

Jo

PS I mis-wrote (2). I meant the backup-light terminal on the transmission, not the brake pressure switch. I don't know this for certain, but that should be a wire attached to the transmission, where it gets very dirty. I don't know which side either, but it should be obvious on ramps or a lift. Again, WD40 and a rag on any transmission electric terminal!

---------- FOLLOW-UP ----------

QUESTION: I am sorry, I made a mistake.  The backup lights work oki but the license plate lights go out when I put the car in reverse.   Thank You for getting back to me so soon.  I am going to work on this problem this evening.  Again, I am sorry for my mistake.
  Thank You again

Answer
Same comment: connections. If you have a test light or voltmeter check whether the plate holder light remains live when in reverse. Same approach for other things like brake lights. If the fitting stays live when in reverse you probably have a ground problem. If the voltage drops, the fault is on the live wiring side. A final thought: is the trunk damp udner the carpet, perhaps causing a leak to ground at a connection? Sorry, can't help much more without the car!