1998 mercury marquis headlight go off and onby themsleves often after using turn signal why?

It's very likely that your Lighting Control Module is failing. I've seen several fail in our Patrol cars although they are Crown Victorias. Same car with a little different equipment. All of you lights are controlled by this module. It's a black box usually mounted under the dash above the gas pedal (although your vehicle is a little older than the cars I regularly service and I could be mistaken.) They are not cheap if purchased new. If you need to, try a reputable Auto Dismantler.

Thanks i appreciate the feedback i'll do that