Headlights don't operate correctly. inconsistantly dimming or going out.running lights going off & on during day driving.Replaced one headlight bulb, didn't solve anything. fog lights,turn signals, backup all work fine.

For more than a decade, GM used wiring for the headlights that barely met the amperage requirements for stock headlights. When owners replaced stock headlights with upgrades, the wires would simply fry.
The only real solution is to run NEW 10AWG (or 8AWG) wires from the headlight switch (or automatic headlight relay) all the way to the headlight connectors. (You can follow the extant routing of wires.) Best of luck to you!

The problem probable is a short to ground on one of ur circuits. when u drive at night u have ur headlights on which forces the alternator to produce more electricity. making it seem normal. or a defective alternator

i don't have a real good answer, soooo just remove the dome light bulb so you can drive it

In the 99 Regal, the headlamp switch is also the dimmer switch for the instrument cluster.
With the headlights on, try rotating the switch to bring up the "nighttime" intensity.
If the headlight switch has a pre-set dimming function for the instrument cluster lighting (in other words, not adjustible by rotating the knob), then
replacement of the headlight switch would be the only way to fix the instrument cluster lighting problem.
See Wiring Schematic below.
Fig.1 - 99 Regal Headlamp switc & Instrument Cluster Lighting

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As a rule you will likely need a new daytime running light module,some GM's need the instrument cluster replaced because the module is built into them,check with the dealers parts department.

no the day running bulbs go out during night driving

sounds like a faulty headlight switch w/dimmer

The day time running lamps are your headlights, the parking lights also conventionally known as the running lights serve no purpose during daylight hours and as such are not necessary in the daytime, and are not illuminated by the DRL module. The DRL (Daytime Running Lights) module illuminates the headlamps in a slightly dimmer state than night time dim headlamps, but does not turn on the conventional running lights. The only way you'll get conventional running lights during the day is to turn them on manually. I just reviewed the wiring schematics for that system, and the DRL relay bypasses the dimmer switch when active and prevents high beam operation, and feeds current directly to the low beam headlights, there is no connection to the parking light circuit from the DRL module wiring.

Does it go completely black or just dim? Most displays dim when you turn the lights on to make the display darker so its not too bright. GPS and other displays do this. Do you have any problems with any other of the car's electronics?

The only thing that all of your problems have in common is the light switch and the dash wiring harness. The stereo is linked to the light switch because of it's illumination feature' You need to test the light switch with a test light by probing the wires at it's plug.