I've got the same problem 86 420 SEL . I think its a dead ground. A Friend told me to make a testing fuse with a blinker and put in place of the fuse. While trying to find the short ( it will click till the short is resolved). This fuse also controls interior lights and seatbelt warning lights.

Haven't had time to work on this yet but soon. if you have any luck please let me know.