SWAMPRV - Acronyms you should familiarize yourself with:
1. PVDM - Photo-Voltaic Display Module (for Solar) - the one in the
display/control cabinet above/behind driver's seat.
2. PVCM - Photo-Voltaic Control Module - the black box that you posted pics of.
3. PVDM Jack - means the cable that is plugged into this jack goes to the PVDM above/behind the driver's seat.
Clicking relay in and of itself means nothing. Under normal conditions, any number of relays in the electrical system will engage/disengage. Usually means it's either charging or done charging. You need to see which of the 4 LED's in the PVCM is lit to tell you the status (red-charge, green-full, yellow-partial, red-low). The PVCM checks availability of PV charge current, by cycling the relay every 3-5 minutes or so(check your manual for exact time checks). If there is no charge current it disconnects the battery to prevent discharge through the PV panels.
"Nothing would make the relay stop clicking" - that's because you won't stop resetting things on/off. Everytime you reset main power or ckt brkrs that interrupts power to the PVCM, you are resetting/restarting the POST routine (Power On Self Test) over & over, hence it never gets to the point where it has finished/stabilized. Usually, if the clicking is fast/furious, a high likelihood that the PV charging current is marginal that it can't decide whether to connect or disconnect until you give it enough time to figure it out on it's own.