I can give a partial answer, to be refined by later research this evening.
The trunk wiring loom, back by the fuel cross tube on the left side contains a wire with a bullet connector, whose marker color is purple I think. (all subject to verification later...) The other end of that wire is, ummm... maybe right by the fuse block... It's been awhile. I think I had to run a (fabricated) harness to and from the fuse block area to the reverse switch on the shifter tunnel assembly (opposite the OD switch). None of my wiring harness was OEM, except the long wire to the trunk area.
Unless someone more informed pipes up, I'll go out this evening and review things...
A basic circuit description is: fused (hopefully) +12V source to Reverse Lamp switch on trans, then back to firewall area, then in wire harness to rear trunk location (by fuel cross-over tube) then on to lamps, with "local" ground. Switch adjustment is via shims. Switch access is possible thru access hole in trans tunnel.
Good luck.