[dallas_]

What he said.

If the item used to work, at least you know it was wired correctly to start with.

If it never worked, you may need to trace the wires and that can be more difficult.

I've found wires that were clipped and wrapped in the wire loom that were never connected. That kind of thing is a bit more challenging to fix.

Also, a wiring diagram is really helpful. Painless and Ron Francis wiring kits are pretty common and if you have one of those the diagrams are available.