like you said, it looks like the audio system is going to be tricky..
do you know if the audio goes through the can bus network, or is sent to the amp as line level, or speaker level?
if the amp accepts line level inputs, then you will only need to get it turned on, and just connect the pc directly to it.
if the amp accepts speaker level inputs, you will need to use a amp-- most of the time, something small, like a motorcycle amp(search ebay) will be more than adequate, and usually cost no more than $50.
if the amp is taking the signal from the canbus network, i can't help as much... i do not know much about canbus.