Truck Repair: Problems starting 1986 GMC Sierra, daytime running light, gmc sierra

Question
Hello:
I'm having problems starting my truck.  It was working fine one day and then not.  I've got no power going to the inside of the truck, I've got no headlights, checked the fuses.  When I turn the key I hear clicking underneath dash near stereo from something that looks like a directional flasher.  
The headlights won't turn on - the battery is good.  I don't seem to have any power at the fuse box.  I used a test light and tested wires (thick red wires) that go into the back of the fuse panel from the firewall. The thick purple wire has no power, I don't know if it's supposed to.  Nothing inside will turn on.  
I have a plow hooked up directly to the battery and that has no problem being powered, the truck just won't turn over.
It's acting like I have a dead battery inside but I know I don't.  All the fuses under the dash are good, the starter is good.  After messing around with it yesterday, I gave up, went out first thing  this morning, turned the headlights on and it worked.  Turned them off, turned the key, and then  nothing worked again, not even the headlights.
Found on the web to check the ignition switch, but if that was gone would that affect the headlights?
Do you have any idea where to go from here?
Thanks in advance for your help.

Answer
Hi Jamie,
The ignition switch should not affect the headlights, except for the daytime running light system, but I don't think a 86 has that.
That clicky thing could be the drl relay. It could also be some kind of power relay... Power windows, blower motor, or something like that.
I would have said a fusible link, but they don't heal overnight. A self resetting circuit breaker could act like that. A fusible link could be blown, and the circuit breaker on the headlight switch is what reset. So look for the fusible links. They look like a fat short piece of wire spliced to a power wire, with a real big splice cover.
The large purple wire is the one from the ignition switch, to operate the starter I believe, so should only be hot in start position.
The block where all the wires pass through the firewall is a possibility for a bad connection, especially if you have road salt and rust problems.
You didn't say if the thick red wire had power....you said you checked it, but the thick purple did not have power.
I believe the red one should be hot.

Van