I think you guys are right, that depending on the year, month, and how well the standard sound system was originally wired probably accounts for this:
"This is really odd because it is happening to cars with standard sound system randomly both with MCU1 or MCU2 upgrade. Most people with premium sound system said the MCU2 upgrade made it sound better. And half the people with standard sound MCU1 or 2 say that the 2 rear door speakers lost all bass and 1/2 the volume, whereas others say it sounds properly with bass and full volume faded to the back, and others say they never had bass and full volume in the back to begin with. "
I might no more next week as they are going to look at my car, but there is no way this is normal. Bass should be coming from the rear of the car, not from the two tiny front pillar speakers, front center speaker, and front doors. Bass in standard systems always comes from the rear, or at least the rear is equal to the front in bass and volume. It is almost like the speakers are wired backwards. And I can't attest to everyone's situation, but before the upgrade to MCU2, I absolutely had twice the volume and bass coming thru my rear 2 door speakers and the appropriate (less) amount of bass coming from my front center and pillar speakers. Maybe one of my cables are on the floor and not even plugged in for bass/volume to rear? There really is no way to explain how weird and low volume it sounds now other than if you were sitting in the middle of a 5.1 surround sound system with four identical speakers around you, and a sub woofer BEHIND you, and even when faded all the way to the back, the sub woofer never came on and the two speakers behind you were 1/2 the volume instead of double the volume. Then when you fade it to the front all of a sudden your sub turns on (remember that was behind you?) and you get full volume and distorted bass out of the front speakers. It's just weird, the 2 rear door speakers really don't do anything anymore, I mean 1/2 the volume of all the other speakers and no bass makes them just about disconnected. Why even have them? Why even have a balance fader if nothing in the rear of the car works? And honestly I have never been in a car with the bass woofer or sub woofer in the front dash or pillars. Having a sub right in your face makes no sense at all, it should be behind you always. SMH
