Initially, it sounds like a grounding problem, but after some thought, I am more inclined to point at the headlight switch or connector.
There are several grounds. There are ground points at the radiator support, near the wiring harnesses a bit before the headlights. There is at least one ground at the rear of the car behind one of the taillight assemblies. There are also several ground points inside the car. ONE bad ground would more likely affect one or two lights - not ALL of them. The one part that is common to all of the lights is the headlight switch. High resistance inside the switch or high resistance at the connector would be more likely to make ALL of the lights dim.
The first test I would make would be to remove the switch from the instrument panel, leaving it connected to the harness. With the lights OFF, check the voltage at the BLACK wire with ORANGE stripe. This should read the same as the battery voltage. Then, turn ON the lights, and recheck the same voltage. If it is still at battery voltage, then check voltage at the other wires with the lights ON.
If you find the voltage drops at the BLACK/ORANGE wire with the lights on, then you have a high resistance connection in the circuit from the battery to the switch. If the voltage does NOT drop, then it is most likely a bad switch.
Once again, I think that ONE ground will normally NOT dim ALL of the lights.
Friday, January 2nd, 2009 AT 9:34 PM