We’ve started having a strange problem when switching the air conditioner from fan speed 5 to fan speed 3 (which we do once the car has cooled down). Nearly every time (which makes figuring it out diffficult), the fan and air conditioner stop for a few seconds. At the same time, the driver information display goes blank, the radio goes silent AND the daytime headlights go off, which I discovered when this happened while waiting behind another car at a traffic light. (Fortunately I haven’t been driving at night under this scenario, so I don’t know if the “real” headlights would also go off!). Our mechanic thought she had figured it out after keeping the car a week and replacing a computer module; but now it’s started up again. while the car may be 10 years old, it’s very low mileage.
I don’t know why the tech would replace a computer module if they had all the clues you stated here. This is obviously a power problem and not a control problem.
It would be helpful to have a wiring diagram of the car. I suggest you purchase a service manual for it. By looking over the wiring print, hopefully you will see a fused line or possibly a power relay, that supplies power to all the areas you mentioned. Then check for a bad connection in that area. If there is a relay in the circuit you may be able to find another one that you can swap it with to test it.
Your mechanic should have, and use, a digital voltmeter (electrical multimeter) and get the WIRING DIAGRAMS from the Haynes General Motors manual #38010. Available at many automotive parts stores, this manual covers your car (and its siblings) from 1988 to 2002. S/he should check the wires going from the blower fan speed selector switch to the blower resistor assembly. The wires could be damaged and shorting to other wires near them. One circuit, a wire may be shorted to, is the power (12 volts) supply wire from the 20A HDLT circuit breaker. It goes to the Turn/Hazard/Headlight switch (IF your car has Automatic Light Control). A CIRCUIT BREAKER can be one which automatically resets itself after a few moments, if the overload is removed. This COULD be happening on your car; but, testing is the only way to be sure.
We got the car fixed. The two suggestions warrant mostly a “wah, wah, wah, waah” rather than a “ta-dah!!” It turned out to be the ignition switch. We’ve not had a single problem with it since that was replaced.