this happened to me all the time with my 98. whenever i hit the turn signal it would turn the radio off, then would come on again. the ground strap on the back was solid, but bad. so i had to run a ground wire out to my engine bay on the firewall, then to the negative terminal on the battery. havent had a problem since.
on a side note, it would send my battery gauge to 14 in the red. weird.