Do as suggested above but also check voltage at the battery, if low charge the battery then check voltage at the battery while the engine is running, should be around 14.5 volts if not your alternator is bad.
Also add your truck specs to your signature.
The wiring is there in your truck. For your '88 look for a single red wire with a black(?) flat connector around the collant bottle. You will have to modify the light if it has the two wire connector. You could grab the harness portion from the yard and attach your red wire to it and ground the other wire if you don't want to cut the connector off the light.
The wiring is different between '89 down and '90 up. For '89 down there is a single wire, the light grounds through the mounting bar. For '90 up there is a two wire connector.