Long story short...rear tail lamps (running lights) stopped working (at night :O ). Brake lights still came on. I tried new bulbs first, and still no lights. I started tapping on the headlamp switch and they started working.
I took out the switch and there was a burned terminal, and the pin in the connector was melted and corroded. It was the white w/ green stripe wire.

My first guess is maybe just an old switch with weak contacts burned the terminal.
Is this common?
What causes this?

 

happens all the time. get a new one. i probably have an extra one.

 

So the switch itself causes it? I also thought maybe a poor body electrical ground would be a cause.

 

A quick search gives you this thread:
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/dash-lights-died-did-research-found-issue-pics-included-850093/

Most informative, as it has some links to Icemark's writeups. It also details why a new switch will not be a permanent fix and the new switch will likely fail in time.

 

MAKE SURE YOU GET A NEW HARNESS AS WELL. they arent that expensive, and most likely you will blow the harness and switch again if you dont.

the problem is that all this power is flowing through a weak circuit.

icemark issued a fix for this, and you need to look to fine it.

same exact thing happened to me.

keep in mind, keep the switch hardware, and the turn signal hardware the same year. dont mix and match years. it wont work. the wireing is different. if you get an s4 switch, get an s4 harness and an s4 turn signal switch. see where im getting at?

 

^ I repair the switches and add relays to the harnesses... The OP can PM me if he'd like to venture into this.

The original harness from Malloy Mazda is $110. I think it's a few bucks more plus some shipping from Mazdatrix... Switches are close to $400, dependent on configuration.

In the case of my neglected 1986 GXL, I was able to resurrect both.
I consider the relay to be a preventative measure. Steps are included in the thread linked above.

 

I was planning on replacing the burned pin with a new one, and cutting off the burned section of wire.

I saw mark's writeup, so I'm going to add the precautionary relay.