wire tracing by using a tone/signal generator
 

If you have no idea where a specific wire leads to, perhaps you could use a "wire tracer/tone generator" kit, to help find it. I've used an AC voltage-only tracer to find a hidden wire terminus, but haven't yet needed a tracer for automotive circuits, but they do exist.

Klein, Power Probe, and many others have them, ranging from cheap to expensive, and while some mechanics like them, others do not (problems with tight bundling of wires or with shared connections). Here's one from Amazon for cheap:https://www.amazon.com/Tracer-Circui...pacebran492-20