This is your answer. Check voltage at the battery. "Click" the switch to get failure, then start following the circuit. Other end of the cable at the relay still have battery voltage? Power wire at the bulkhead have voltage? Etc. get the wiring diagram and trace the circuits and you can quit throwing parts at it. At some point you'll find where there ISN'T voltage. Don't forget that the problem could be on the ground side, so keep your meter grounded to the battery.

My brother-in-law had a FWD Charger that kept breaking clutch cables, like 3 or four in a couple of months. I was home on leave from the USAF and took a look at it. Opened the hood and asked him if the trouble started when he replaced the ground cable. He was lazy and grounded the battery to the body instead of the engine. It ran fine, but when the clutch was pushed in and the starter engaged, it was running all of the amps through the only piece of metal attached to both the engine and the body, the clutch cable. Heated the slug end at the fork up enough that it would snap. WIERD stuff can happen with bad grounds...