How do I hook an amp/sub up to my stock radio? Can I run a sub off the rear channels of my head unit?
Yes, there are two common ways to do this. Some amplifiers come with “speaker level inputs” where you run the wiring that was going from your stock radio to your speakers into the amplifier. Another method, in case your amplifier doesn’t have speaker level inputs, is to buy a Line-Out Converter (or LOC for short). An LOC is a whole lot like an extra speaker level input module; you run the existing speaker wires from your stock radio to it, and then you connect the LOC to the amp using RCA cables. Both methods get the signal to the amplifier, and then you just need to run new wires from the amplifier to the speakers/subwoofer.
New to this board, all the info you have supplied is awsome. I have a sub box, amp, and the line out converter. Can you tell me for an 06 STI, were I can find the rear speaker wires to tap into for the LOC, and what color's the wires would be. Greatly appreciated.