The idea of using a MID located in the glove box can make sense, assuming you have somewhere to plug it in. Sometimes when an aftermarket radio is installed, they use an adapter harness and plug into the stock radio connector. Or worse, hack off the stock radio connectors, and just splice into to wires they need.

If you're lucky, the aftermarket stereo was installed using all new wiring, as would be done in a high end installation. (Running new RCA cables to amp(s) in the back, new speaker wire from amps to speakers, etc)

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2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp