[~The Jester~]

Not sure on a '90, but here goes. There's a large black wire with an orange stripe, comes from the connections at the starter relay, goes into the cockpit. Has a fusible link in it. Check for loose connections, or a bad/weal fusible link. Sometimes the factory solder on those links gets kinda cruddy. Don't hold me to the wire color, on my car the black/orange feeds half the fuse block (headlights, dash, radio) and the big yellow one feeds the other half, as well as the rear defroster and a few other things.

Good Luck!