its a 79 250 and the reverse lights don't come on at all, i ve replaced the switch next to the tranny that enguages the lights but nothing happened, any ideas please.

 

Easiest first check would be a blown fuse. Next, condition of bulbs and sockets they fit in and ground connections. They may be burned out or the socket connectors and bulb bases or grounds could be corroded. After cleaning and bulb check, use a test light to determine if you are getting power to the socket with the shifter in reverse and the key switch in the "on" position (wheel chocks, please and parking brake applied). If no power to the socket, time to check for juice at the newly installed reverse light switch. If fuse, bulbs, sockets, grounds, and new switch are all good but still have no lights, you have a break in the wiring somewhere that will need to be repaired. Find it by tracing the wires from one end to the other in that circuit. After saying all this, I'm assuming the transmission linkage is adjusted properly and allows the switch to activate in relation to the shifter position?