Here's the wiring diagram you can reference, I shamelessly stole it from autozone.com
http://repairguide.autozone.com/znetrgs/repair_guide_content/en_us/images/0996b43f/80/21/19/77/large/0996b43f80211977.gif
Everything in the top left corner is applicable to headlights. I suspect there may be an error in the diagram, and terminal H should really be connected to the Head pin of switch B1 in the Headlight Switch. You can ignore the Autolamp module and relays for now, as they can only turn on the lights when they're switched off, they cannot turn the lights off when the switch is on.
From that diagram, it looks pretty straightforward. Power comes from Fuse 2 in the distribution Box, goes through the headlight switch, to the dimmer switch. From the dimmer switch, power goes to either the high-beam or low-beam sections of the bulb. Finally, the other side of each lamp is grounded.
So, step-by step troubleshooting procedure: You will need either an electrical test meter, or a 12v test light. Unplug the DRL module, we're ignoring it for now.
1. Verify that power is coming into the headlight switch. Pull the headlight switch connector. You should have 12v measured between pin B of the the headlight switch connector, and ground.
Result: If you don't get about 12v, suspect Fuse 2 in the Power Distribution Box, or the wire between Fuse 2 and the headlight switch connector (or possibly the power distribution box itself, but that's a big mess you hope you won't have to get into).
2. Verify that power is coming from the headlight switch to the dimmer switch. If everything's good in step 1, then inspect, clean and reconnect the headlight switch connector, and pull the dimmer switch connector. You should have 12v measured between the Red/Yel wire of the dimmer switch connector and ground, when the headlight switch is on.
Result: If you don't get about 12v, suspect the headlight switch, the headlight switch connector, or the red/yel wire.
3. Verify that power is coming from the dimmer switch to the lamp itself. If everything's good in step 2, then inspect, clean and reconnect the dimmer switch connector, and pull the headlight connector on the driver's side headlight. You should have 12v measured between the Red/Blk wire at the headlight connector, and ground, and it should turn on and off as you flip the dimmer switch. You should see the same between the Lt Grn/Blk wire and ground, except when one wire is powered, the other is off.
Result: If you don't get about 12v, suspect the dimmer switch or dimmer switch connector.
Note: If you only get power on one wire, and never on the other, then one of the wires, or that pin in the dimmer switch connector, is bad. I don't think this is the case, as you would have either the high-beams or the low-beams still working.
4. Verify that the headlights are good. Although it would take a minor miracle for both filaments in both lamps to fail at the same time....I guess it's possible. If you have an ohm meter, you should measure between 2.5 and 4 ohms between the pin that the Red/Blk wire connects to, and the pin that the black wire connects to. This measurement is made on the headlight itself, with the connector pulled off.
Note: If you don't have an ohm meter, but can verify that the lamp works by applying power to it directly, you can skip this step.
Result: If you read 0 ohms, then the lamp is shorted, and you probably blew a fuse already. If you read significantly higher than 4 ohms, or if you don't get a reading at all, then the low-beam on that side is burned out.
5. Verify that you have a good ground. If everything's good in step 4, then inspect, clean and reconnect the headlight connector. Sneak your probe in to the pin that the black wire connects to, without removing the connector entirely. You should measure 0v between that pin, and ground.
Result: If you measure voltage there, then you have a bad ground. Follow the black wire to where it connects to ground, and inspect the wire all the way down. Unscrew the black ground wire from the frame, clean the lug and the frame at that point, apply some dielectric grease to the lug and frame, and screw it back down nice and tight.
If you've gone through all these steps, correcting problems as you find them, and at least your driver's side low-beam headlight doesn't work, then some kind of electrical miracle is occurring. This process tests every single part of the circuit between the power distribution box, all the way to ground.
If you have any questions about a step, or the results of a step, post it back here. I should be on hand to reply to you within a few minutes.
And, as always, let us know what you find!
Spott
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