tail lights and int. light no worky

When i turn the headlights on, the tail lights dont work and all the interior (radio, clock, guage cluster) lights shut off. When i turn the headlights off, all these lights will continue working. All the fuses are good. Im thinking it is a short or a loose ground. Any ideas on where to start looking would be much appreciated as i have no manual to help guide me through the wiring.
Thanks!

Anyone?

What year and style is the car? Sounds like a short or could be wires crossed. Have you replaced and switches? Check the wiring on any switches that you've replaced or that you think may have been replaced. I once crossed the assessory and constant power wires on my radio and it cause all kinds of problems with my all my lights.

It is an 88 Mercury Sable. I havent replaced any wires or switches....this problem just came about out of no where. I am thinking there is a short somewhere just dont know where to start looking.

I've got a manual and a scanner. Let me get the pages on my computer and I'll email them to you. Maybe they'll help.

Hayne's manual is about $20- and they usually include wiring diagrams. Start with the headlight switch. Use a test light or a meter to check for voltage.