Basically, there are very few mfgs. that offer a schematic to end users. Yes, a signal generator is nice to have but you can make a DIY device that will help trace circuits. This device could be a light, a buzzer, or a bell. If you can source a 12V bell or buzzer you can clip that on with two alligator clamps at the end of the DC circuit and then mess with the power source to trace circuits. Also, your distribution panel may be marked with the devices that it powers.
Inspecting wires at the distribution panel and where Mr Rodent has made his modifications is a "must". It's a shame that the wire mfgs have gone to a soy based wire jacket as rodents love that little treat when nesting or living close to them. I've read of owners that have opened the hood of their truck to find that whole wire assemblies have been chewed to bare wire. It can be a mess.
Good luck on the 12V repair.
