The reverse lights on my 94 5-speed don't work either. The reverse light switch is located on the driver's side of the tranny and can be replaced pretty easily. I checked mine by removing the wire harness for it and using a jumper to short the two wires-- with the jumper in place the reverse lights came on, so I knew the problem was at the switch. After replacing the switch though, the lights still didn't work.
The previous owner told me the truck had a re-built tranny installed (don't know where or exactly when), and I now believe that the tranny shop didn't install the internal mechanism that activates the switch correctly.