Im working on a 89 super dutys headlights. Driving down the road they will be on then flicker and go out and come back on after a bit. Ive replaced the dimmer switch, headlight switch and a big boxy relay under the hood on the drivers side attached to the fender. What else do I need to look for to stop this?

 

Undo clean maintain reassemble all ground wire connections. Ensure body is grounded, remember it's on rubbers and dirt and rust can affect earthing through mounting bolts. Battery earths. Engine ground to frame and batteries.
Now work on connections in your power cables, junction boxes including any on the bulkhead. Look for frayed wires around cable ends. Google fitting headlight relays, these articles have great info on cable runs vs voltage loss vs headlight output.

 

Wrong forum. This is the 80-86 forum.