I would start at the trailer light connector if you have one, these can get corroded and short out, if it isn't in the bumper look under the rear, it may have been added and is just hanging under back of truck.
At night I would press on the brakes and see if front parking lights glow a bit, meaning bad ground.
Do the same with turn signals, each side, see if opposite lights glow.
Also check Reverse lights, turn them on and check for glowing front bulbs
Turn on parking lights only and make sure all work and at the same brightness,
Then try it with head lights.
Alot, if not most, of these circuits run thru the Multifunction switch(turn signal arm), so try jiggling it and see if a fault occurs.
The 2000 lighting also uses a microprocessor and relays so you may need a replacement.
I would do as many test as you can think of and write down results.