2001 ML NAV/Audio Problems - quiet sound/can't input addresses ever
 

So the car is a 2001 UL with NAV/ML and it's got a couple issues. First, out of the four "corners", only the front left speaker(s) play at regular volume. The rear door speakers and the passenger side front all play very quietly (the balance/fade setting isn't the problem). The subwoofer also doesn't seem to do much of anything, or if it does it can't be heard. Would this be a problem with the amplifier or the head unit/source?

Second, in the NAV, the address button is greyed out all of the time. From reading the forums I understand that it's supposed to do that when the car is moving, but it's always like that in this car, even with the car in park. I saw a few forum posts about this but no one ever posted a solution. The other day the NAV system said it didn't have a DVD, ejecting it and putting it back in solved that but that's a little worrisome. Could the NAV unit in the trunk be going bad? If so, I found a YouTube video showing how to "upgrade" the older NAV unit by replacing it with a newer one from an 04/05. They said the newer version "has a few extra bells and whistles" but didn't say what they were. Does anyone know the difference, and is it worth upgrading the unit?

Third, am I correct that the only aftermarket options are either the BeatSonic kit that puts a double-din head unit in place of the tape deck/CD player unit and requires discarding three of the wood trim pieces in the center console, or a Chinese Android head unit that replaces the NAV screen and bezel? If so, for the life of me I can't figure out why nobody has made a single din kit that just replaces the tape deck/radio controls while leaving the factory wood bits alone. If I was any good with fiberglass I would absolutely make one myself. /end rant

Any help is greatly appreciated.

There are a few writeups here from members that have installed a hidden single DIN where the CD player is while retaining the wood panels. Definitely a possibility, not much fabrication required except slight enlarging of the opening. As far as upgrading, even the newest nav is nearly 20yrs old and being on disk it will be severely out of date. My nav went out too and I replaced it simply because I didn’t like the error screen I was getting, even though I replaced the head unit with a double DIN that has nav on it.
As far as your sound issue goes, I know the amps in these are prone to failure but I can’t say for sure if it’s your amp or head unit. From my understanding the amp in these does most of the work related to the audio and the stereo is more of just a display with no real audio capabilities, it just sends all signals to the amp for processing. All tuning, fade, etc is done by the amp. Hopefully someone corrects me if I’m wrong but if had to guess I’d say it’s the amp.

Yeah I looked at that option, it's neat but I don't like the idea of having to drive around with that door open all the time, or having to always open and close it to mess with the radio. The center console looks so nice with it closed. I also think the amp is likely the issue. From what I've read it seems the head unit only sends a two channel, fixed volume signal to the amplifier, then like you said, the amp does the rest.

Any ideas about why the nav button for entering an address is always greyed out? Or is that an uncorrectable fault that eventually happens as these cars age?