I have a 1987 Dodge W250 that I’m working on and there is clearly an issue with the wiring. When I try to turn on the headlights only the dash lights and the lights on top of the cab come on. I discovered that the left turn signal light wasn’t connected so I plugged that back in. After connecting the turn signal light I tried to turn on the headlights the dash lights came on and the right turn signal light turned came on on the dash and when I tried to turn on the left turn signal all the lights started flashing including the dash lights and work lights and both turn signal lights

 

find and clean every ground you can; mopar loves to hate its grounds loL!

 

Dose you 87 have the headlight dimmer switch in the floor? or is it mounted up in the multifunction switch in the column?

 

Yeah, welcome to Mopar's Electric Nightmare (no, not an Alice Cooper cover band)
Start by checking all your wires going thru the bulkhead connector. They are prone to corrosion so Mopar filled them with grease but that washes out.
Check the fuse box connectors and fuse clips for the same.
Next check all the ground wires you can find.
Does the dimmer switch do anything?
It's been a while but it sounds lie one of the 12 volt feed wires might be broken.
hope this helps.