If I was doing this same project today, I would surely go for something like this to drive the audio from the carputer to the factory speakers:
Believe me, it will avoid some headaches in case of eventual manipulation by somebody not involved in your installation.
I spent the last 2 afternoons fixing the installation because people at the dealer disconected everything (I have already taken the CPU out and only left the cables) to fix the AC and then they were unabale to connect anything back in the right position.
I've discussied with my audio expert about the chance of modifying the installation using one of those huge military connectors to drive all the cables through just one Plug&Play harness.
I will keep you updated if we finally decide to go for it.