Help! My 1969 CST Won't Start

[Swampdawg]

My 1969 CST truck sits up a couple of months at a time. This time it won't start. I took the starter off and had both starter and solenoid checked out and they are ok. When I take the ground cable off the battery and touch it back to the post , I hear a clicking sound in the regulator. I have no power whatsoever... no lights... no fan motor on the heater. I cleaned both battery post and cables and the terminal junction block on the fender well beside the battery. The small hot wire off the battery goes to that junction block and then from there to the regular.

This problem is very hard to find! Any ideas. What should I try to replace first?
Best Regards,
Swampdawg