rca splitter cable

I have a 5 speaker system ready to install and my after market head unit only has front and rear channels. I have a single 12” sub and my amp has rca inputs for the sub. I want to use a “y” rca splitter until i buy a head unit with all the rca outs i need. So which speaker should I split from...

I have a Pioneer DEH-X3500UI (DEH-X3500UI - CD Receiver with MIXTRAX™, USB Control for iPod®/iPhone®, and Pandora® Ready | Pioneer Electronics USA), which has 2 RCA outputs. I am currently using one of those for my Subwoofer amp input. I want to put in a 600 watt amplifier for my door speakers...