After three separate times of ripping out my trunk lining, and trying two different radio modules... I'VE MENDED IT!
A quick backgrounder for those who missed my previous post:
Many, many moons ago my radio began cutting out intermittently. It wasn't a pressing problem in the sense that it often came back quickly. An irritation at best.
Fast forward to later this past summer and it began cutting out entirely. Usually, I could pull the fuse and put it back and I would get the radio back again for awhile. It seemed to be confined to just the FM radio and AUX was unaffected. But after a dozen or two times of going through this dance, the whole thing cut out entirely. This was mildly annoying, but as headphones and bluetooth speakers existed, I put up with it.
This pretty much sums up my experience at the time.
Then, sometime around November, the radio began making an absolutely demonic screeching noise whenever it was 'on'... and I say 'on' in the sense that no sound would come out but if I pushed the power button, it would stop. But sometimes it would come back on its own... like I said, demonic. So, this time, I pulled the fuse permanently, continuing to exist with my bluetooth speaker and the irritation that electronics bring to driving older and high mileage cars.
The final straw for me came just before Christmas when the long-non-functioning pre-wired bluetooth module decided it would like nothing more than to beep at me, loudly and intermittently, followed by VERY LOUD prompts to "speak the name," "speak the number," and "system off" that I could not turn off or reduce the volume of. GREAT. This action inspired the following two reactions from me: first was to send me off to the fusebox again to pull yet more fuses, and the second was to fill me with a resolve to finally solve the damn thing.
I'd deduced from some extensive googling that the likely culprit was the BM53 radio control module located in the trunk next to the amp, but I was having a hard time sourcing a part. eBay had options of course, but very few of them were both reasonably priced and came with any sort of guarantee of functionality. I resolved to find one at a local junkyard, but after visiting a few pick-n-pulls I realized that not many BMWs of this era came equipped with the up-level radio that had the BM53 module and the ones that did must've been quick targets for opportunistic business-minded scrappers because every single car in the yards I visited lacked the part. I did grab an amplifier from the trunk of a similar era coupe as mine just to be sure that wasn't the issue.
While my junkyard excursions were mostly fruitless, and the amp wasn't my issue as suspected, I did get to familiarize myself with the process of removing the trunk trim on these cars before I worked on my own. To be honest, if you're even slightly skittish about screwing up your car, this is a great way to practice something before trying it when it counts.
At this point, it was back off to eBay to try my luck at finding a functioning BM53 module without breaking the bank. I knew that BMW and Becker had made these modules from the late 90s through 2006, slowly adding more functionality to the radios over the years. The early cars were kind of basic, and things like station and song ID, and more importantly for me, AUX input were added in the later models. Knowing this, I wanted to be sure I got a unit from a newer car, so I looked at the various eBay listings and pictures trying to find one that had a date code in the 2003-2006 range. I settled on a listing that looked promising:
As far as I could tell, this little alpha-numeric was likely a date code for the week and year of manufacture. Alas, when the part arrived at my doorstep after New Years', the one I received was quite different, being manufactured in the 40th week of 2001 rather than the 7th of 2003 as the photo in the listing states. (the one my car came with had a date stamp of 12/03) Still... better to try it and see what happens right?
A couple hours and a small amount of swearing later, I had a working radio but no AUX input. Worryingly, my weird and annoyingly loud bluetooth beeps and instructions continued on as they had before, so I had to pull the radio fuses again and carryon with my portable speaker setup until I could try a newer radio module. However, all was not lost! I had confirmed my theory that it was indeed the BM53 unit, but the hands-free phone prompts were weird. I was very excited to have a working FM radio again, even if I couldn't plug my phone in just yet, but the hands-free instructions were so annoying I couldn't use it. I did a bit more googling there and came across some hands-free instructions which reminded me that BMW had decided it was a good idea to put the hands-free speech button behind the cupholders and BELOW the folding armrest, which I always keep down. This gave me a new theory: in my car, this area under the armrest had become a depository for stacks and stacks of those blue fiber masks I get at work, and I just stockpiled more and more of them there each time I was given one, to the point where it was possible my arm resting on the armrest could be compressing the stack of masks to the point of pushing down and holding that very button.
So I tried removing the pile of covid-crap and replaced my radio fuses yet again, and the glorious, glorious sounds of FM radio once again massaged my ears! Another mystery solved.
At this point, it was back to eBay and try to find a newer part so I could get my AUX input back and truly have my radio functioning as well as BMW intended in 2004. I ended up finding a listing with an actual tag detailing the make, model, and year of vehicle it was from: a 2005 330i sedan. Not my year, not my bodystyle, but slightly newer, so it should work. Parts cannon: reloaded.
A few days later, with unexpectedly rapid shipping, I swapped the parts out this morning and it works! I also swapped back my original amplifier back into the car, as that wasn't the issue. Everything functions perfectly, just as good as it ever did... well, except for the broken / burned out pixels on the display and the SatNav that only works 5% of the time, but who gives a crap about any of that, my radio works again!
I actually have functioning audio! I'm over the moon!
TL;DR: Keep a sharp eye on your eBay listing details, and persistence pays off!