When you say your tail light goes off when your headlight comes on, does that mean the tail light works when the headlight is off??
Do both elements of the bulb work if you take it out of the light and connect the bare bulb terminals to a battery?
If it was working before and it has just stopped working of its own accord then it means either the filament is bust or the supply to the filament is disrupted somewhere.
Take a look at the bullet connectors in the wires under the rear mudguard (fender). You may find them loose or broken and it's a simple matter to clean and renew the connection.
New to the forum, i've got a 2000 ttr250 with a baja kit on it. Love the bike, i've had it about ten years now. Have a question about the tail light. My tail light has stopped working when the head light is on. Does the tail light and head light run off the same wire? Brake light works just not runnig light. It's a new bulb also.