Adramelech,
I'm not 100% clear on the problem. You have a '66 GMC and you can't get the backup lights to come on when you put the truck in reverse.
I don't think you have to have any other light on for the backup light to function. They have there own circuit..they don't go through any other switch, only the switch that closes when the rig is put into reverse.
Does this truck actually has separate backup light?? You haven't mentioned what type of transmission you have, automatic or standard.
You say you have replaced the reverse backup switch. If you have a circuit tester, you need to verify power on the line side, then verify you have switch closure with power on the load side.
If these prove good, I think the search would be on for broken wires or wires to the wrong place.
Make sure the bulbs are OK and the lights have proper grounds. With a little inexpensive circuit tester, this should be pretty easy to troubleshoot. Good luck and keep us posted on your progress.
Stuart
