The manual, page 6: http://mediacdn.jlaudio.com/media/mf...pdf?1346971846
It doesn't have convenient speaker level inputs, but you can probably get away with splicing the speaker level wires onto RCA plugs. Radioshack sells a few different kinds.
EDIT: A better question is, why bother? Your stock amp is in the trunk, presumably where you're mounting the new amp, thus giving you access to both line level and speaker level signals.