Ok, so I abandoned ship on trying to tap into other, existing 12v
wiring and decided to run new wires direct to the battery (which fixes the problem). I'll chalk the original issue up to a bad or weak ground.
Next question: I'm gonna have to run a marine grade duplex wire along the drain and water lines to get it done. Anyone see any issue doing this? I had 12v
wires running in my boats bilge without issues. Can't see it being a problem here.
Someone above asked which heater I bought. It's the Dickinson Marine P12000.