My wiring diagrams show that switch indeed carrying a hot wire right from the #12 fuse. It must be grounding out inside the transmission or somewhere along the wiring harness that runs first from the tranny up through the dash/console, and then along the driver's (left) side floor back to the driver's (left) side taillight and then over to the passenger's (right) side taillight.

The wiring diagram indicates that it's a solid black wire that heads back to the taillights from the switch, but that probably doesn't help much as there are likely more solid black wires.
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85 245 DL - 91 Mazda MX6 Turbo

I had the same problem last year. The wiring for the reverse lights had fallen down on top of the drive shaft. Once the shaft shredded the insulation, the circuit grounded right there. I would blow a fuse anytime I shifted into reverse or second. A new section of wire and some cable ties made everything good.
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Jim - '92 Red 245 "The Radio Flyer" - '85 245 270K "Old Girl"

There's a switch on the tranny. Check the wire to it, maybe it's somehow messed up and touching a ground.

Wiring diagrams not available to me just now.

Good Luck,

Bob

:>)