I see all these threads about hooking up computer sound card's to existing head units and/or speakers, and I can't figure out why everybody wants to do it the hard/expensive way.
There's a cluster of wires that goes to the speakers which plugs into the back of the head unit. Find out which wires go to which speakers and splice into them. It's free. It's fast. And it works better than any transmitter or modulator since it's a direct connection.