I FINALLY had the time to figure all this out successfully...
And I didn't have to custom-build wiring from the mic back to the modules in the trunk, like a few threads here and on other forums did when a Bluetooth MULF module was already in-place (e.g. 6FLA USB/Audio Interface package), nor modify any of the connections or wires at the modules themselves.
After scrutinizing the electrical schematics for a Bluetooth MULF setup versus a TCU setup then realizing they both use the exact same wiring, connector junctions, and pin-outs, the key breakthrough came when I was relentlessly searching the internet for similar issues and came across a
BMW Universal Hands-Free Kit Upgrade, Installation Instructions 01 29 0 430 250, for a
1-series E82! And there it was on page 11, step 6, under "All Cars:"
This priceless bit of information was missing from all BMW E90-specific literature and DIY guides.
After removing the trim panel and glove box, connector X14087 was in plain sight:
I disconnected the female connector on top and confirmed that the wires coming down from the mic were in the disabled pin locations of 4, 5, and 6:
The male connector on the bottom was correctly wired (according to BMW's WDS schematics) with yellow, black, and grey wires at pin 1, 2, and 3 respectively, with nothing at pins 4, 5, and 6:
I used a jeweler's screwdriver to undo the pin clamps:
Re-pinned the yellow, black, and grey wires to positions 1, 2, and 3:
Snapped the pin clamps closed and reconnected the plugs together. A successful telephone call out verified the mic was now fully functional, and a sigh of relief was let out!
It's probably a safe assumption that all US-spec Bluetooth equipped MULF's only need to re-pin X14087 and some INPA magic to have a fully functional hands-free system; if a TCU is co-installed with a Bluetooth MULF, then just a re-pin with no coding is probably all that it takes because all my telephony functions worked "out-of-the-box" before I coded (except the mic, of course), although I'd still recommend coding to ensure no issues arise later.
I'm not sure why BMW positioned the mic wires in "non-functional" pin locations (perhaps a devious way to prevent customers from easily enabling hands-free functions?), but at last I can finally close this project on my "To Do" list...
