You should be getting 12+ volts at the wire in the plug. Here's what I would do, I would start with the ground wires, and test for conductivity through the wire. Use your volt/ohm meter, put the leads on each end of the wire. You should see 0 resistance. If you are getting resistance, your wires are getting corroded/weak. Alot of factory wiring has scrimped ends, not sealed ends. Moisture and and dirt gets in there and causes issues, attacking the copper wires. Alot like when your battery terminal gets all corroded. I'd start with the ground wires and work my way back. Testing each and every wire that goes to the headlights. Another thing to do, is to check where the ground bolts down too. Make sure they are clean.
