Hey guys, I've got an 87 Ford f150 thats having really odd electrical problems.
tested for a short...none.
good battery, good alternator.
the truck drives just fine without the headlights on, if i turn the headlights on the internal lights and the headlights will flicker and the truck will die. Any idea where to start looking at this point?

 

Check your ground cable. Also how do you know the alternator is good? You need to see what kind of voltage it is charging with while under load I.e. when you have your headlights on, blower fan running, and wipers. Very possible it puts out enough to charge the battery while nothing else is on but when you start putting more load on it or can't keep up.

 

Grounds grounds grounds. Just put a new engine in my bronco and started having all kinds of problems like key off door open and relays, injectors, and iacv was clicking so grounds can do some very crazy things with these older trucks.

 

Whats really odd is while driving home all the lights went out, but the truck was still running. Got home, went to start it again...dead. Replaced alternator, voltage regulator...now theres no horn. Where do I go from here??