This is my first amp hookup, but I thought it was standard now for head units to have one pair of RCA's for the front channel outputs, one pair for the rear channel outputs, and on the higher end head units to also have another pair for the subwoofer channel output.
Those wiring possibilities are listed in my amp's manual. And I do see where you can split off one pair of RCA's to feed both the front and rear channels if that is all you have. And the amp can be selected to use only the front channels to feed the subwoofer channel (5-channel amp) or combine both the front and rear channel inputs for the sub channel, if you don't have a separate pair of subwoofer RCA outputs on your head unit like me.
I thought it was strange that one pair of my RCA's would be at the end of 6" of wire, while the other was built right into the back of the head unit without any wire leads. I wondered if one pair was for an input too, but I remember reading a post about how people have wired up an auxillary input through the CD changer DIN port, so I figured it was unlikely that my old head unit already had an input when other slightly newer Clarions didn't. So I took a chance last night and got out my old-school boombox that has a line-in mode where you can plug in RCA inputs on the back. I plugged in each set of my head unit RCA's to the boombox line-in one at a time and then faded the head unit to the front and rear channels to figure out what each set was for or if they didn't work at all.
I was worried that because they didn't share the same ground (boombox on house A/C ground, and head unit on the boat ground) that it might not work or might even scew up the head unit. I just turned my key to the left so no other dash accessories would be on. I rationalized that if I blew the head unit then I could get a new one.
But it worked. The pair of RCA's on the 6" of wire are for the front channels, and the ones built into the back of the head unit are for the rear channel.
I see the antenna input on the back of the head unit, and the large round DIN connector for the CD changer that could also be wired for an auxillary input. The only other thing I don't know for sure about the head unit is a large black female connector on another 6" black wire lead coming from where the 6" grey RCA front channel RCA's are coming out of the head unit. But I'm guessing that is for a wired remote. It has 6 slanted female hole/slots in it at differing angles. I don't need to hook up a wired remote anyway, so I guess I'm all set and will be hooking up the amp today.
Thanks Mike and Craig!