Reverse light switch
 

I have a 99 Isuzu trooper-manual transmission. My reverse lights do not work and the bulbs are fine. Someone mentioned that it may be the reverse switch on the transmission. I've looked for it but haven't found it. Where exactly is the switch located? and what would it look like?

Two ways those switches get mounted, one is on the side somewhere, the other is internally. Look for some electrical wires coming out of the transmission somewhere, most likely somehwere around the top. Check for continuity along and between them.

On automatic transmissions, this is often somewhere around the shifter, but on manuals, you would be looking for wires coming out of the transmission somewhere, probably attached to a rubber plug that may or may not be large enough to pull out. Don't pull the wires so hard they break off though, as the plug may have something internally holding it in. The reverse light switches are usually so dirt simple they never go bad for decades, but something may have happened to one of the wires leading from it, so that is really the first order of business to check out.

A continuity check between the bulb sockets and those wires with the truck off but in reverse might tell part of the story, then the trick is to find that break in the wire.