Don't know about the rear panel....... Mine's working so I never messed with it.

 

The newer hhr models received a stronger sub. Older models had an anemic sub in the rear. With the bass up and putting your hand over the rear sub you can feel it moving. But sitting in the seats the 6.5 over power the output of the sub. I used to run bass boost on an ipod and it helped with the factory system. Sometimes you could hear the rear sub with the fade favoring to the back with the eq on head unit set on jazz and the right song playing.

There have been a few guys that have added a shallow mount 8" to the stock box. Or made their own in the stock location. I think they went with an aftermarket amp. Most have just added their own subs and amp. I unplugged the pigtail to the stock amp and am running multiple amps, speakers and subs in a sound quality setup.
BTW the stock speakers are 2 ohm. Installing a 4 ohm will cut the power received by the speaker in half. Running 8 ohm will cut the power received in half again. Each reduction of power by half equals 3 db reduction in loudness.