Several warning lights come on intermittantly (mostly on) and when that happens, the interior fan (htr/def etc) stops working and then there is a loud beeping coming from behind the inst.panel. The warning lights that come on are: ABS Brake system, Brake system warning light, and Service 4WD. There is nothing wrong with the brakes or 4WD, all checked by a certified brake/transmission mechanic. What is this causing (electrical) problem

You could check if you have front wheel speed sensors. they should be located in the front hub assembley, check for wires running along control arm. Additionally, the two first warning are refer at ABS Module, so, I suggest scanned to know exaclty which fault code is stored there. Some Autozone or AutoShop with reader codes scanner could make it.

How old is your battery? Some vehicles turn on this dash lights when this occurs and its time to new battery; if this its thru... don't forget that our vehicles disable the charging system for a few seconds until the idle stabilizes in cold weather. What this means is that a marginal battery may provide JUST enough juice to start the vehicle but not enough to keep it reading all the inputs and outputs from the PCM, TCCM and EBCM. It then throws multiple trouble codes.

It's simple to have a battery load-tested. (Usually free at any battery dealer.) You cannot tell the state of a battery by a simple voltage test; it must be load-tested.

Now, about heater/defrost, I suggest check the blower fuse, then the blower resistor pack if you've got the manual controls, then the blower motor itself. If you've got the digital controls, check the fuse and the blower first. If those are good, it's probably time to replace the control module.

If your blower only failed to work at the time of the indicator light flakeout, it's most likely the effect of the power being held back from other systems in order to allow for starting. If that's the case, it doesn't come back immediately after the truck starts. There's a lag while the charging system takes over and the PCM can allocate power to those additional sytems.

Additionally, I found this article and you could visit your dealer and ask about this TSB... I don't understand it all, but it does point to the problem: bad ignition switch.

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Subject: ABS Red Brake SVC And 4WD Light On HVAC Blower Inoperative - keywords B1375 C0379 DTC HI4 ignition IPC L04 MIL on U1000 U1041 PIT3832C (12/04/2007)

Models:
2004-2007 Buick Rainier,
2002-2008 Chevrolet Trailblazer models,
2002-2008 GMC Envoy models,
2002-2004 Oldsmobile Bravada

This PI was superseded to update Recommendation/Instructions and model years. Please discard PIT3832B.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s).

Condition/Concern:
Vehicle may exhibit a combination or all of the following symptoms: ABS light on, Red BRAKE light on, Service 4WD light on, HVAC blower motor inoperative. Various DTCs may also be present.

Recommendation/Instructions:
Diagnose for loss of battery voltage to the rear fuse block on circuit 300 ORN for the following fuses:

Note: Prior to proceeding forward with the information below, it is important to verify the voltage as indicated however it is equally important to verify the integrity of each circuit. This can be achieved by performing a voltage drop test or by adding a device to each circuit that will supply sufficient load to verify the circuits integrity.

HVAC 30A, 4WD 15A, HVAC 1 10A, BRAKE 10A, TBC RUN 3A.

Check circuit 300 ORN for battery voltage when the ignition key is in the RUN position.
Check for battery voltage at the rear fuse block under the rear seat Connector C1 pin D9.
If battery voltage IS NOT present at these locations, check for voltage at the ignition switch pin G for circuit 300 ORN.
If battery voltage IS present at the ignition switch pin G, check circuit 300 for an open.
If battery voltage IS NOT present at the ignition switch pin G, replace the ignition switch. Note: This PI does not apply to the Saab 97x. The 97x has a unique ignition switch location and an electrical harness. With this new switch, the electrical center pin designations and the ignition switch connector is different. In the event the concern above is experienced in a Saab 97x, circuit 300 can be checked for integrity using the appropriate schematics in eSI. Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
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