You really don't have the proper electrical system to be powering a cap. If you had a proper H/O alt and a couple batteries then maybe... But for now just ditch the cap.
I don't think ground loop isolators work for pioneer issues. You just need to wrap some bare wire around the outer jacket of the rcas on the deck and ground it to the deck chassis.
What kinda of wire is that? I love when people put surprises in boxes lmao. Like Richard at TechForce. Sent me a sock, aspirin bottle and a zip tie LMAO!
Stick the postive and negative leads together and see what reading you get and subtrack it from the reading at the sub. And for dual 2 2.3 sounds right.
Battery probably doesn't have anything to do with it. I don't recall if I even heard this but isn't there something with some cars if something is messed with it sets off the anti theft?