2 preamp outputs, front and sub

Hey folks, I need some help with wiring my components to the amp. I want to install front and rear components but only have 2 sets of preamp outputs. Front and Sub. I have a Kenwood 8405 4 channel amp, and Alpine CDE126BT. I currently have my front components (C2 650) running on the amp but im unsure of how to connect the rear speakers, since i'd have to run another set of rca cables to the amp but the only place to connect them to the receiver is in the "subwoofer" spot. Will this still work? and to make things complicated, i want to run a subwoofer later on, how do i make it all work? Thanks for your help...beginner, but like doing the work myself

oh yeah, the forum says its a good idea to post your vehicle. 2007 Silverado Ext Cab. New body style no bose, but do have Onstar. thanksbeginner, but like doing the work myself

http://vault.alpine-usa.com/products/documents/OM_CDE-125BT+126BT_EN.pdf See Bottom of page 33. When Subwoofer is turned off, that set of RCA jacks become rear full range output. For later, when you connect a sub, you will have to turn the sub back on on the radio and use the front RCAs to feed both the front and rear channels of the amp. You will lose the ability to fade front to rear. If you insist on being able to fade, take a GOOD look at the CDE-147BT. 3 Sets of RCAs, 6 channel time alignment, 9 band true parametric EQ. It retails for 200 dollars.

How do i use the front RCA's to feed both front and rear speakers? thanks for the help btw.beginner, but like doing the work myself

If the amplifier does not have a 2/4 channel switch, you may have to use 2 RCA Y adapters. You will need Ys with a 2 male and 1 female configuration.