1988 MR2 20 Valve reverse light problems...

Hi, I own a 1988 Toyota MR2 20 Valve and I am having some issues with the reverse lights. Although I have rigged up a switch under my instrument cluster to turn the reverse lights on and off the lights will not engage when shifting into reverse. I have about 30,000 miles on the JDM 20 valve and the rebuilt 5-speed transmission has about 2 or 3k. I had the transmission technician check to see if the gears which activate the lights were meshing and he told me mechanically that is all fine? What else could be the problem? A switch or fuse or something else? If so, could someone please tell me where I can locate these areas? It has been an annoyance for quite some time now, thank you very much.