Truck Won't Start With Key -- Help!

[72Much]

Ok guys here goes. I have a 72 C-10 with a 307/350 set up. I'm trying to get this truck back into service. Here's what I've done so far.

I try to start the truck with the key and nothing but a faint "click" when I turn the ket to start. The headlights come on with the switch. The solenoid on the starter has two wires, a yellow and a purple. A test light on the yellow wire to the solenoid says it has power with the key in the "on" position. The purple wire shows no light when in the "on" position or when the key is turned to the "start" position. I took the starter off and to a local store to be checked and it tests good. I went ahead and changed the solenoid thinking that it wasn't allowing the start switch to complete the circuit and it didn't fix anything. I was able to use the 'ole screwdriver from the positive battery cable over to the purple wire post to get it to start that way but my last attempt on that just clicks and sparks. I don't know what else it could be. On a last attempt I took a wire from the purple wire directly behind the ignition switch and ran it out of the cab and under the hood to the purple post on the solenoid and it still won't start that way. I'm at a loss!

Also, I found one source of the "click" noise generated with the key. It is coming from a small black box on the passenger side of the windshield wiper motor under the dash. I can actually feel the click in this box. Does anyone know what that box is? Could this be the problem?