Josh, tracing electrical problems is really pretty easy if you have the right tools to do it. Get yourself a Volt/Ohm meter from Radio shack - they are real cheap and last a long time if used correctly. I will help you trace out the harness etc. I have several used harnesses as well as some good rear wiring setups. The only way the backup light can flash when you put on the turn signals is if you have the wires crossed somewhere. Not uncommon with 30+ year old truck that have been screwed around for trailer lights etc. It also helps to have a wiring diagram out of one of the truck shop books----every wire is color coded but there are some that are duplicated so you kind of got to know what goes from where to where.
