Ok, I have been warned 84s are known for ground problems. Seems my interior lights work sometimes and sometimes they don't. It always seems to be affected when you drive the car and go over bumps. So I assume it must be a bad ground. Where do you suggest I start trouble shooting first?

 

Not just 84s, most early C4s. Not sure if there the same as an 87, but I'd look first at the ground connection on the back of the drivers head (several grounds on that one). On an 87 there is a digi ground on the drivers side post. And sometimes it's the gound wire on the digital dash itself, or the connection to the digital dash that cause problems.

Might be a write up in the Tech section, worth a look.