New HU install - Amp problem?

[shinner]

So I put a new Kenwood DDX812 in my 2006 Camry yesterday. Used a Metra harness that integrates the factory JBL amp.

Volume from the front speakers is fine but from the rear, it's probably 1/3rd of what the front is. I don't remember any imbalance using the OEM HU. Using the on-screen balance and fader controls, I've verified that the sound is being directed correctly to each side and front to back (i.e. I don't have the RCAs plugged in incorrectly).

Trying to figure out what my line of troubleshooting should be...thinking of opening the dash back up, plugging the OEM HU back in and running through balance and fader to make sure I didn't somehow screw up the amp. If that all works, I don't know how to prove out the Kenwood HU's output. I can always use the Kenwood's normal speaker wires to the output connector of the amp (essentially running directly to the speakers themselves) but that wouldn't prove anything out with regard to the pre-outs on the Kenwood.

Any thoughts?