I took my car to Fry's electronics to have my sub/amp installed and they tapped into the speaker wires going to the sub for the line out converter.
Hope that helps.
This thread had a lot of great dialog and wiring diagrams. Was able to tap the speaker and the amp power-up right from inside the trunk, just had to run the main positive line from the battery to get clean power and found a solid ground at the seat bolts as I recall. But with the wiring diagrams it was a snap for the LOC.
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