My wife's car has an issue that the DRL warning light flashes and the DRLs do not work. When the headlights are turned on the DRL warning light goes off. Also, none of her interior courtesy lights work but all the dash lights work fine. There is also no buzzer for the key chime. Her trunk light does not work either. Any thoughts?

A UK car not USA not canada. bingo now known.
DRL can be super hard to fix.(diagnose)
1:get a shop to fix it, drive there in day light.
2:check all fuses?
3:scan the PCM (bcm) for DLR errors .

the DRL on some cars have SUN sensor on the dash
clean off the dust there yet? hummmmmm!
some cars this sensor as a know adjustment called sensitivity covered in the glove box users manual;.

You might want to start by checking the fuses associated with your headlights. If those are fine, check the headlight relay. You can do a search online to see where the relay is located on your vehicle.

Symptoms point to the cluster microprocessor. How was it proven? See diagram below.

There's headlight relays for both low and high beam (if I recall correctly) and a main relay (on the junction box near the battery). I'd guess initially that a relay is faulty.

If its the daytime running lights (DRLs) that are flashing the high beam on and off, they are controlled by a combined oscillator and relay box (looks like a large relay can). This box provides about 9v to the HB lights to provide the DRL function. You can disconnect the DRL box and see if the problem goes away.

HB and LB are separate from the DRL circuits. If the HB or LB are flashing (ie at night), I'd suspect the main relay.

SD

The DRL module may be defective.

the sensor on the dash may need to be replaced a local dealer should be able to turn these on and off with the computer good luck

Hey, I'm having a very similar problem. Did you figure it out?

check the wiring under the carpet and to turn off the day time running lights just pull the drl fuse

Check your DRL relay that is located in the fuse box. Also check the connector under the battery box for corrosion. (Not a smart place to put a connector) GM really messed up with the wiring on the 98-00 cavaliers headlights.