Lots of options for sure.
1.) Speaker Level or High Level inputs directly to the amp. This is usually an adapter harness that comes with the amp that you can splice directly into a speaker run to pickup the audio signal.
2.) Line Out Converter (LOC) can be used to accomplish the same as above for amps that don't have direct speaker level inputs. The LOC will convert the speaker signal to a Low Level (RCA) connection. Then you just run your RCA's to the amp and connect to the appropriate low level input on the amp.
3.) An OEM integration unit similar to what Steve is talking about with the JL unit. This will wire up similarly like an LOC, picking up speaker level inputs and dishing out RCA's but depending on the unit you get there can be endless possibilities, from a glorified overpriced LOC, to a top of the line audio processor. I know the JL Cleansweep was/is pretty popular with OEM integration, its basically a LOC, but it actually calibrates itself to your stereo by removing any factory processing and EQ'ing that might be in the factory signal so you have a flat signal to start with.
