It is going to be different based on the amplifier and you can compromise the performance of the system based on the approach so here are a few options Best, Better, Good ...

1. Using an Infinity or JBL amplifier you have the benefit of Balanced Differential Inputs which allow you to run your factory amplifier directly into the amp either over RCA or high level inputs. You can also terminate the outputs of the factory amplifier with RCA ends and run them to the amp.

2. go to Becker or other source for the pre-amp line level outputs and feed the amps that way.

3. Go out and purchase some good quality hi-low level converters to convert the amps output to RCA. Note these devices have serious roll off issues at the bottom octaves and upper end so this is a place where generally it pays to spend a bit more if the performance is there.