Additional question/comment: The previous owner didn't use the RCA level input (even though the HU has 6 ch output), he had the HU hooked up to the speakers through another Metra harness (I believe it is the Metra 70-1770). Do I use both the line level and RCA inputs, or just one or the other? How does the front and rear tweeters get signal? My thought is the RCA plugs were only for the woofer amps in the back of the car. How does the front, under-dash amp, receive signal for the tweeters?