We had exactly that symptom on our '86.

Problem was a loose ground wire on steering column.
Don't laugh.
I spent a ton of time replacing headlight relay and other junk before I found the loose ground wire.

Remove battery ground wire - since
the '91 likely has airbag for driver.
- IF you have original radio, it will lock when you remove battery.
- You'll need code # to get radio working again.
Remove plastic shell covers on steering column.
Ground wire in question was slightly forward of the turn signal lever.
Black wire (I think) held down by a chunky screw.
Check any other visible connections while you're there.

Other things I fooled with - check these if the ground screw check doesn't fix it for you:
Hi-beam relay - on fender wall, just to the rear of battery.
Our '83 - '86 - '89s have, behind the battery (front to rear):
#1: hi-beam relay, #2: thin black thing, #3: blade fuse for fuel pump/ignition

Headlight relay, mine is to the left of brake pedal, under the carpeting.
Bad wire insulation is possible, but not that likely on a '91.