Do a voltage drop test from your main feed wire and the ground wire. Connect a volt meter from the battery + to the hot wire under the dash and turn on some load, sounds like turning the key on will suffice. If you have a voltage reading on the meter it is telling you you have a large resistance to electrical flow in that circuit. Same thing with the ground side. What you will be reading is current that is flowing through the volt meter instead of through the circuit, electricity is lazy that way, always looking for the easy way out. If it weren't so useful, I'd swear it was a supporter of our current government. But back to the point, you want little to no voltage to show on the meter. If you do show a reading then try and isolate where it is by doing the same test at different points. It is really quick and easy and foolproof.

You are looking for less than .20 volts DC on your circuits, with lower loads that number should be lower still. anything over that have too great a drop. This test can be used to check battery terminals as well, one lead to the post, one to the clamp or wire coming off the battery. You should see nothing at all there for voltage, even under a high load, like cranking the engine. The higher the load, the better this test is. You must have current flowing for this test to be valid.