Electrical gurus, help!
 

My 3rd brake light and reverse lights haven't worked since I've had the car. All fuses check out ok. Are there certain grounds I should check/clean?

grab a volt meter and see if you're getting 12v to the bulb socket itself when the car is put in reverse. Red probe to the center of the bulb socket. Black socket to any bare metal in the engine bay.

Isn't there a reverse sensor/switch on the transmission? Perhaps that is broken or not connected.

Yeah, tried that, and there's no juice to the bulbs. The bulbs themselves are all good...

Yeah, it bolts onto the driver's side of the transmission, towards the rear. It has two red wires coming off of it. Mine was bad on my '91. Cheap part and not too hard to fix.

Sounds good, I'll check that out. Probably a simple single pole switch (check continuity after putting it in reverse)?

yep. exactly. it's probably just dirty and worn out.

I was going to respond with checking out the switch at the tranny, ^^ guys beat me to it, but that IS a good place to start. Maybe the 3rd brake light wire harness has a connector in the trunk that's unplugged? I have never looked to see if it is powered by a disconnectable harness but that's a possibility you might want to check out. Could be a broken wire where the harness flexes at the lid also.

the reverse lights are controlled by the switch on the trans housing. They are fairly exposed and probably fail easily. Also, polarity doesnt matter on them

The wires to the third brake light tend to break at the trunk hinge location. Everytime the trunk lid is opened and closed the wires will flex and eventually break. It's happened on all three of the Miatas I've owned plus at least two others belonging to friends.

Check the wires at the point where they join the harness next to the trunk hinge.

Barry