Damn, the bass is pretty damn good on this Sure Electronics board. Before the mod, I didn't know that my speakers are in such a bad shape. After the mod, I think I have a couple of the blown speakers whenever I played the song with bass. Damn, more money into the bucket LoL.

Indeed, ground loop can be formed with BT adapter. There is a good reason why optical cables are used inside E60 to provide galvanic isolation. Though neither ground loop nor RF interferes would explain the rolled off bass Boost7 was experiencing. If you operate with presence of RF interferes (microwave, 2.4GHz access points and wireless cards, other BT devices, wireless keyboards/mices) you are likely to hear audio dropouts when buffer gets empty. You may hear pops and clicks along the way before audio is muted. Also audio quality will degrade as the bit pool setting will be changed and data rate will be reduced. Though non of these would affect the low end extension (frequency response down low). You may notice reduction in dynamic range though.

Try to reduce the volume on your phone by couple of steps from the max. I am not familiar with Sure Electronics board but the high output voltage will clip the front end (ADC) on the Logitech 7 stereo. For instance most of the phones will clip the front end if volume is set to max. At 10kOhm if you play 1kHz tone iPhone will generate about 1V 0dBFS (full scale signal) (Logitech BT adapter does exactly the same) that most definitely is going to clip the front end. This is likely what Boost7 experienced and I forgot to mention this. Most pop/hip-hop music is recorder really hot (close to 0dBFS) so it will definitely clip the front end.

Do you think this might be the case of improper grounding that I am experiencing? since I am grounding it to the screw that holding the Nav DVD and CD player. If that's the case, what is the proper way to ground without interference?

Proper grounding is a huge topic, and we will derail this thread if we go in depth here. I would try to tap to the same 12V and ground as your NAV DVD and CD player. Looks like you have ground end covered though.
If it sounds like your speakers are blown when you stream via BT only (not CD/Radio) then you are overdriving the front end. See my reply above on this matter. Try reducing the volume on your phone and see if it improves. A lot of Bluetooth devices don't have volume sync implemented (typically works only with iOS devices, for instance Android has dropped volume sync support according to coworker who writes code for BT devices). So to get the best signal to noise ratio you would want to keep the volume as high as possible. Though with Logic 7 it may not be an option as it may clip the front end.

Thank you bro, will try and do that.

That was the #1 reason I went with it. Everything else including RCA AND 3.5 mm jacks, APTx, small size has been bonus. It's why I started this thread. Couldn't rightly keep this mod to myself.

sweet

Awesome mod! I would like to do this but im not very savvy with whats connects what, is there a way you can create a print up of what wires goes where? Schematic i think thats what they're called. Thanks in advance!!

This may help: