My park & tail lights go out when I turn on the head lights

That happened in my 1999 Buicke Century. Replaced the bulb 4 times kept burning out all within a week. Finally replaced the blinker socket and no more issues.

Go to http://www.bbbind.com/free_tsb.html free wiring diagrams ! You having tail light problems ? Do you know how they work ?

They are turned on by the BCM !
Park, Tail, Marker and License Lamps
The park, tail, license, and marker lamps are turned ON when the headlamp switch is placed in either the HEAD or PARK lamp positions, or anytime the automatic light control (ALC) turns the headlamps ON. When the headlamp switch is placed in the park lamp or headlamp positions, ground from G202 is applied through the signal circuit to the BCM indicating the park lamp request. In response to this signal, the BCM applies ground through the park lamp relay control circuit energizing the PARK LP PCB Relay. With the relay energized, battery voltage is applied through the switch side of the relay, both park lamp fuses, and the supply voltage circuits illuminating the park, tail marker, and license lamps. Ground for the left front park and marker lamps is provided at G101 and the right front park and marker lamps at G100. Ground for all rear lamps is provided at G302.
Turn Signal Lamps
The LT T/SIG and RT T/SIG fuses located in the underhood fuse block supplies battery positive voltage to the body control module (BCM) for turn signal and hazard lamp operation. When the turn signal switch is place in either the LEFT or RIGHT position, ground from G202 is applied through the turn signal switch signal circuit to the BCM indicating the turn signal request. In response to this signal, the BCM applies a pulsating voltage to the front and rear turn signal lamps supply voltage circuits cycling the lamps ON and OFF. The BCM also sends a message via GMLAN to the instrument panel cluster (IPC) to cycle the turn signal indicator ON and OFF depending on the position of the turn signal switch.
Ground for the turn signal lamps is supplied as follows:
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G101 provides ground for the left front turn signal lamp

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G100 provides ground for the right front turn signal lamp

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G302 provides ground for the left and right rear turn signal lamps

The dash lights are on the same circuit as the tail lights/park lights. If the front park lights are not on as well, look for a blown fuse . If fuse is ok. possible headlight switch/relay/wiring problem

Two possibilities. The head light switch or the turn signal switch. All the exterior lights run threw the turn signal switch. I am assuming you already checked fuses.

Check fuses and head light switch.

Yes it sounds like the switch is stuck in the park position.
You could pull the harness plug for the switch and check the circuits with a meter, or replace the switch.

Could be the headlight switch.
The switch has two sets of contacts for the parking lights, the park position and headlight position.
You may need to check the circuit at the switch to see if you are loosing power when the switch is moved from park to head.

see the diagram and fix it.Test the fuse,lose connection, dirty, corrosion, oxide.continuity.
God bless you

I'm checking into that same problem for a early 90's buick ( it's a GM also ) , I've checked the obvious ( bulbs, flasher,fuses,etc) all where fine. I assume your front turn signals work if you manually operate the turn signal switch or put on the hazard switch? The Buick's rear lights won't come on even with the hazard switch on. When you say one tail light has been out for some time, do you mean the brake/signal light or park/running lights? The Buick was like that with one side out for a while ( brake/signal light) but both the rear park/running lights work. Now, I've narrowed the problem down to the turn signal switch. I'd bet money on it. I just have to dig out my steering wheel puller, bite the bullet and get 'er done. You should be carefull if you're attempting that repair yourself ( AIRBAG ), use a manual. ( You could do a quick test by trying the turn signal while you pull the bottom of the arm toward you,move some of the wiring in the column near the arm,etc. I did and the signals started working. Could be the switch,dirt or a broken/loose wire. there's a lot of wiring going through the column and a lot of movement ( turn signal,high beam,cruise, then add tilt) Hope this helped.

Typically this is caused by a bad dimmer switch or a connection at the plug to the dimmer switch.