I'm on the road for work this week. Drew a Chebby Impala for a rental car this week with the Hertz Never Lost. They have a new version system with a GSM cellular radio in it for live updates. Every time I start the car I get the "bzzt bzzt tat tat bzzzzttt" off the GSM radio into the stereo speakers as the unit syncs up.

If your interference sounds like that I am not sure that even Bluetooth will help you. That crappy GSM modulation is the RF equivalent of throwing a boulder into a swimming pool. The harmonics that come forth from that couple into anything with two wires and a semiconductor junction and a speaker. (Ever built a crystal radio as a kid and notice how it picked up every bit of stray rf and demodulated it, so you got the Tower of Babel in your ear?)

I think that is why the owners manual for the W210 cautions against running any type of transmitter inside the vehicle that was not the MB installed cell phone with the external antenna. It's not coincidence that the built in cell antenna is in the back bumper on the W210, as far away from the powertrain and ABS ECUs as possible, and 90* rotated with respect to the main wiring runs in the car.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech