New safety switch, new bulbs, new taillight/backup light harness, fuse is good - still no backup lights! How does battery power get to the switch? Should I have continuity when I check the backup light plug at the switch? Stumped!:confused:

Power is supplied from the fuse box...which if its good there will put +12V on one side of the neutral/reverse safety switch....when the shifter is moved to reverse, it should close the contacts on the switch which sends the +12V out to the bulbs. The other side of the bulb is ground.
The switch should have continuity between the terminals when in reverse.
If you unplug the wire to switch, one side of that terminal should have 12V with respect to ground, the other terminal should have continuity between it and ground (this checks the circuit from the plug back thru the bulbs, if its open, starting looking for a broken wire)

Ok, thanks for the input. I'll give this a try.

If you've also had problems with your other rear lights... sometimes the problem isn't the power, it's the ground. Check for ground between the lamps and the box. If that's good - here's the kicker - check for ground between the box and the cab.

A friend of mine had to run a ground wire between his box and cab just to get his taillights on.

- Sylvia

My taillights, license plate light, side marker lights, etc. all work it's just the backup lights that are a problem. However, I did have the bed off for refinishing so I'll check that cab to bed ground thing. Appreciate it.

Cab to bed?
There is no need to do that.
Bed to chassis ground...and a cad to chassis ground, but there is no reason to need to bond the cab and bed together.

If all your other tail lights are working, I really doubt that its a grounding issue. sounds like you have a broken wire somewhere outside the cab.

Solved! I chased the electrons as advised and found out the aftermarket trailer wire plug that I was using was defective. While this plug seemed like a good idea at the time there is apparently some flaws within the plug and affected the backup lights and on occasion the left brake and turn signal light. This plug installed between the wiring harness from the cab and the tail light/brake light/back up light harness just before the rear bumper. I may try another one since this is really a slick way to set up for the trailer lights. Anyway, thanks for the help.