Headlights work but not taillights what is the cause and how do i fix it?

It's an electrical - 'earthing' fault. Quite simple to fix, but you'll need an auto electrician

Check the fuse for the tail lights it is not the same fuse for the headlights.

Sounds like a relay is bad. The best place to have any electrical problems checked is at the dealer. call and set up an appointment and they should fix it.

Change the bulb. When you press the pedal the brake switch changes the circuit to the 21w brighter circuit for the brake light.

It is a ground problem. When you turn on your headlights, they are sharing the same ground that your brake and T/S lights are using. Check all ground connections to the body from all headlight and taillight wiring are connected and tight. If it doesn't affect your taillights, it may be in your T/S switch. Check your ground connections first.

THere is a short in the dashboard switch for the lights. Change the switch or just remove it and find the short!

GOOD ADVISE--I ALSO LEARNED THAT THERE MAY BE FUSES IN UNDER THE HOOD ALSO. FOUND OUT TODAY THAT IF THE DOME LIGHT DON'T WORK THEN IT'S FUSE COULD BE BLOWN AND HINDER THE CIRCUIT TOO. I'M NOT GOOD ON VOLTAGE READING , BUT DO HAVE A TEST LIGHT. IF NO FIRE AT THE COMMON TAIL LIGT-DASH LIGHT FUSE , THEN THAT MIGHT NARROW IT DOWN. I HAD THE SAME TROUBLE IN MY 1990 FORD XLT LARIAT AND WAS BLOWING THE TAILLIGHT FUSE. IT WAS A SHORT IN THE EXTERNAL TRAILER KIT HARNESS WHICH CAUSED FUSES TO BLOW REGUARLY. WHERE IS RELAY FOR TAILIGHTS AND OTHER FUSES UNDER THE HOOD??

look for a ground wire that wasn't installed after the repair or a broken ground wire from the impact

The fuse to the taillights/running lights?
It can't be a grounding problem if both taillights and dash don't work.
If only taillights don't work, it would be suspicious grounding.

You'll have to use a 12vdc test light and start probing for power with tailights on, and I'd work from tailight & go forward. You did check all your fuses right?