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On the inside of the lid is a diagram .
The headlamps may be turned ON 2 different ways. First, when the driver places the headlamp switch in the HEADLAMP position, for normal operation. Second, with the headlamp switch placed in the AUTO position, for automatic lamp control (ALC). During ALC, the headlamps will be OFF in daylight conditions, or low beam operation in low light conditions. Battery positive voltage is supplied to the low beam relay and the high beam relay in the underhood fuse block. When low beam headlamps are requested, the body control module (BCM) supplies ground to the coil side of the low beam relay. This activates the relay and the relay supplies voltage to the left hand low beam headlamp through the LH LOW BEAM fuse and to the right hand low beam headlamp through the RH LOW BEAM fuse. Ground for the left hand low beam headlamp is provided at G101. Ground for the right hand low beam headlamp is provided at G102. When high beam headlamps are requested, the body control module (BCM) supplies ground to the coil side of the high beam relay. This activates the relay and the relay supplies voltage to the left hand high beam headlamp through the LH HIGH BEAM fuse and to the right hand high beam headlamp through the RH HIGH BEAM fuse. Ground for the left hand high beam headlamp is provided at G101. Ground for the right hand high beam headlamp is provided at G102.
For the low beam problem, Swap relays 45 & 46 in the underhood fuse panel. Relay 46 is for the Low Beams. If that fixes the light, replace the relay that cam out of position 46. Since the high mounted brake light is working, that indicates that the brake switch is good. You need to check the bulbs in the rear tail light assembly, if they are good, check fuse #34.
If it is anything like my 04 GMC Sierra, there is a relay panel under the hood. In that underhood box where you found the fuses, there are also relays. They pop out (the diagram is on the lid of the box). After the GMC hit 200k, the relays started going out. The high beams are on a relay, the lowbeams are on a different one. You can pop one out and replace it with the horn or foglight relay if you want to check it out. I am betting that the DRL bulb just happened to go at the same time. Mine go out regularly..I would say around every 10-15k miles. I don't know if this helps, it is just something that I have run across with a similar vehicle.
Easy fix, swap RELAY # 45 with relay #46 next time your lowbeams aren't working, should have lights. 2 things, clean the contact on the relay OR replace it. Very very common problem. Easiest way to clean it is to take it out and plug it in several times. This worked for me, but I keep a spare just in case.
The dimmer switch on the stalk usually operates a relay in the fuse box. It could be the dimmer switch, relay, lowbeam fuse, or odd coincidence both lowbeam filaments went out. Use a test light and work backwards.