You sure it is not the ground? If I remember correctly, which I may not, I think the whole tail light housing is the ground which can cause problems if you switch the style. It has been a year since I did mine so I dont remember 100% As far as the switch, that is on the side of the transmission and can be tested with a wire. The lights themselves are just another fuse.
Complete rebuild axle/engine/everything swap in progress:
Parts already gathered for assembly: 2001 Ford 302, 1990 Mustang GT 5.0 ECU and wiring, NP435, NP205, Pair of axles from 04' F350 SuperDuty (Sterling 10.5 Posi rear, Dana 60HP Front-Both Disk Brakes 3.73 Gears), 37" H1 Tires, 9" lift TJ Coils for 2 link conversion in front, Stage 2 Clutch Kit, 3 Row Aluminum Radiator, Chevy Astro Steering box (4" front Stretch), XJ Rear Leaf Springs (6" Rear Stretch), and lots more.