I have a motherboard which has an optical S/PDIF output or 1/8". I'd like to "split" that signal into the appropriate channels so that I can then connect that to the wires behind my car's headunit which, in turn, run to the amp. The factory Bose amp just takes a single connector with a million wires running out of it, so that's why I would need to separate the signal into separate channels. On the other end there are four RCA connectors: front left, front right, rear left, rear right. The sub-woofer signal does not require an additional connection.

Edit: Revised to include S/PDIF or 1/8".