that will only be good for 2 channels. It's been attempted, but I've never seen a successfull run on the attempt of using that loc from the source through RCA wires to the high level side to the speakers. You wouldnt be able to get enough power to the speakers that way.
Maybe if you tried a line driver but those are only built for adjusting a signal before it goes to an amp. I've never attempted it that way.
What I would suggest is to use the LOC, you would need 2 of them one for the fronts and one for the rears. Hookup the LOC to the cherished stock radio so you'd have RCAs out from there. There are some switching devices out there that would allow you to switch from one source to another. Or it could be built from a serries of 8 relays if you want to get difficult.