ALT HUM when right channel RCA plugged in

[Sleezyk]

So I buy new Sky High Car audio RCAS and when I plug one of the channels in I hear a alternator hum through the subs. The other channel does not do it. AND NOW THE CRAZY PART... The Hum Goes away as soon as I plug the other channel in.

I can even just touch the outer metal to the RCA input and it goes away.

Does my Head unit have a bad RCA jack or something? It works fine as long as both are plugged in but really weird. Its like one channel is not being grounded.

SETUP IF IT MATTERS:

Rockford FOSGATE P1000BD AMP

SUNDOWN SA-12's

Ported BOX that I made myself based of sundowns requirements.