AUX input sound quality issue

[llajumpvid]

I've been noticing lately that when playing the same audio source through the DVD input the sound quality is much better and louder than the AUX. The volume is a good 3 clicks louder and both the DVD and AUX have a PIE line amp on them. My AUX left channel doesn't sound as good as the right either. So I figured I had a connection coming loose. I switched the left and right channel inputs and have the same problem which tells me the problem isn't the RCA cord so it must be the connection at the AUX jack.

Now here is what has me stumped. When I unhook the common ground connection that I have connected behind the dash on the AUX jack my bass goes wayyyy up. It's not instant but over about 3 or 4 seconds it goes up, and the instant I hook the common ground back up it goes back to crap. I expected the audio to quit when I unhooked the ground but obviously it’s getting ground elsewhere. Anyone have any idea what phenomenon is going on that would cause the bass to increase so drastically when the common ground is unhooked. I'll probably solder the wires and maybe run a better set of RCA's but from what I can tell my connections are good. Manipulating the connections have no effect.

Does anyone know what the circuit on the AUX jack does? I unhooked the audio from behind the dash and ran it through the jack itself and I have the same audio issues, but I didn't unhook the physicall connections behind the dash(I didn't have anything feeding through them though)....

Suggestions or thoughts on what might be the deal.
Here is how I have it wired.