tail lamps /then brake lamps but no tail lamps
 

1998 DiscoI -When head lamps/tail lamps are switched on @ dark ,brake applied ,tail lamps go out and brake lamps come on.Never have both. checked bulbs /ok.whats problem?

Usually that is caused by a bad ground.

ok ,where would I start looking for that problem.I carried it to an Auto Electric shop and they said the Tail lamp assembly needed new ones. I doubted that seeing both were not opperating.I could be wrong.