like i said, i'll do a full write-up with pictures over the next week or so...but right now i had to put the car back together and tuck the fm direct adapter away because the cable on it just isn't long enough. i have it tucked into the passenger C-pillar right now until the 2.5mm extension cable i ordered from amazon comes in on wednesday.
a while back, someone opined that the connector on both the antenna amplifier and the radio unit itself (not the one that comes directly off the glass, the other one) looked like a FAKRA connector. i spent a while doing research and eventually found that it seems to be a FAKRA-B (white) connection, as the different FAKRA types are both color coded and lettered. the kicker, though, is that there's a light blue FAKRA-Z connector which is universal and fits all of the other FAKRA connectors.
the other day, i finally found these on amazon:
https://www.amazon.com/gp/product/B07PJX5611
https://www.amazon.com/gp/product/B07PHW5J6C
these are your antenna adapters that fit the connections in the model 3 perfectly and will allow you to connect the FM direct adapter. the catch is that it will only cut the signal completely if you disconnect the other connection to the actual radio unit (the ribbon cable and black connector)...i don't know how those two work together, so i'm not sure if there is anything hurt by disconnecting this. i can say though that even with the FM direct adapter in-line using those two adapters above, and the black connector disconnected, it seems to work just the same as it always has when the XM unit is turned off (which is what is supposed to happen with the FM direct adapter in the first place). i'm still able to use the FM radio if i turn the XM unit off, just as intended. to be honest, it does work if you leave the ribbon cable connected...just not as well. there were certain strong stations that the XM unit would not override with the ribbon cable connected, but once i disconnected that it was able to override everything (even the strongest stations).
i haven't noticed anything different, but i haven't really tested it fully yet other than a real quick drive around the neighborhood...i guess it's possible though that there might be a loss of range or something similar by disconnecting that ribbon cable. i won't be able to fully test it until the 2.5mm extension cable comes in, because like i said the one that is attached to the FM direct adapter isn't nearly long enough. for testing i had it stretched across the middle of the car and that wasn't even long enough to reach where i have the onyx plus mounted...i had to pull it out and put it in the cup holder.