We have a 74 k5 it has an 83 front clip on it, we just got the truck a few days ago and last night we were driving home (first time we drove it at night) and we lost our tail lights, brake lights, and dash lights so we pulled over and it blew the 20 amp fuse so we replaced it and no sooner did we start driving it blew again. After changing it 5 times (all within about a mile) we gave up threw the hazards on and drove home. We have the entire dash apart and have looked for a loose wire that may be grounding out but have not found one, anyone have a suggestion of what else we can try? Do the switches go bad and cause this maybe?

Also when we are driving the heater is stuck on hot but with the switch to off it still has hot air coming out but when you try to turn the heater to on the blower motor doesn't come on and it looks like there are several stray wires around the unit in the truck and under the hood, does someone maybe have a photo of what that should all look like so we can compare it and fix it? Any help would be greatly appreciated thank you in advance

 

Sounds like you have a wiring nightmare on your hands. You're going to have to figure out where/if splices were made under the hood to mate the front end harness from the '83 to the '74's pass-thru into the interior. The wiring problem for the lights is much more likely to be under the hood than under dash.

On my '74, my fan is always on whenever the ignition is on. There is no true off position.