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.
The optical S/PDIF output carries a digital audio signal that requires a decoder. Note that S/PDIF is just a physical connection, and the digital audio signal could be PCM (2 channel aka stereo) or Dolby Digital (5.1 channels) or some other digital audio format. The typical device for converting the optical output to multichannel analog audio in the home environment is called a preamp/processor, or just pre/pro. An AV receiver (a pre/pro combined with multichannel amplifiers) could also be used. For the mobile environment, you should search for a basic Dolby digital decoder like this one (not a recommendation).
Your revised title/question now makes no sense. The S/PDIF connection is multichannel and digital. The 3.5mm connections (what Keltari/you are incorrectly referring to as 1/8") are discrete analog channels that don't even need "splitting".
Multichannel analog audio can be output on many motherboards if the mobo integrates such a chip from vendors as Realtek or Analog Devices. Some mobos conveniently have six (stereo) audio jacks on the back panel, whereas others use the conventional three 3.5mm jacks and require SW configuration to define their functionality. Two front (left & right sides) channels, two rear/surround channels and center + subwoofer channels can be made available on just three stereo (3 x 2 = 6) jacks to provide analog line-level 5.1 outputs.
If you do choose to use the multichannel analog outputs, then instead of directly connecting each line output to an audio power amplifier, insert a (multichannel or ganged) passive attenuator (like this premium one) between each PC output and amplifier input. This would provide a quick manual master volume control instead of relying on just a GUI mixer or the individual level controls on each amp.
Sawdust's answer is correct, however you have another option:
If your motherboard has a 'line out' 1/8" jack, you can use that to connect to the left/right inputs on your car's head unit. Do not use a 'stereo out' or 'headphone out' jack without checking to see if they are amplified (or if your head unit can take the amplified signal).