Hi, and welcome.
I don't get what you mean by splicing the cables from the motherboard. If you have onboard sound, you need to get some mini-jack to rca (tulip) cables to connect the sound to an amp.
A standard amp that is! My Volvo had an amp too, but that was a positive ground one, which kept blowing my fuses when connected to something else then the stock radio.
You also could get a fm-modulator, but you will lose on soundquality.
How do you use the stock amp now? Did you connect the line-level outputs of your clarion hu to the amp or is it unused now?
If you connected your clarion, you will be able to connect the pc the same way, maybe needing some switch to choose between hu and pc.