Even if they are connected in the HU,that doesn'tmean they won't pick up some EMI on the way to the amp. I've seen it happen before. My suggestions is to try one, if you can get by with just one then great. If you can't get by with just one, I'd do em' all.

 

correct me if im wrong, but ive always heard that if you buy cheap rca's and run it next to the power wire to the amp you will get engine noise....so i always ran rca's on the opp. side of the car, and i never had that problem....ive owned a few pioneers before and doing this i never had a problem

 

Well then you've taken good care of your pioneers

 

so to make the alternator noise go away all you have to do is ground the rca's. I have 4 ga grounds on both of my amps and I have ohmed out my neg cables from my battery they are fine, but I can't get rid of that damn noise, just to be certain do I need to ground them all or one per pair the pic showseach pairgrounded together and to the chassy of the hu That works????????

 

there's numerous reasons alternator noise can come about
grounding RCAs is just one way..