Just bought a 1985 535i a week or so ago. I'm trying to keep it looking original but add some modern amenities, namely Bluetooth music streaming.
I bought an "FM modulator" - had no idea these existed before. It sits between the antenna and the radio, and when powered takes RCA audio input and outputs it on an FM frequency to the radio. There's lots of generic ones available for cheap, like this one:
https://ebay.io/m/fH8nEG
Then I got a 12V powered hardwired bluetooth receiver. Also cheap and generic, like this one:
https://a.co/d/09THroRa
I wired the red wires from the FM modulator and the Bluetooth receiver together and then spliced the white/violet 12V accessory power wire running to the radio. Did the same with black but spliced the brown wire for a ground.
It works decent for ~$30. The volume output of the bluetooth receiver is horrible so I'm gonna install a line driver between it and the FM modulator.
Lastly, grabbed a fog light switch (part 61311368893) since it fits perfectly in the blank switch spot next to the right of the radio. Pretty common choice I think when people add a switch to control the power antenna. The FM modulator only passes through antenna signal when off, so a convenient way to turn it on and off is necessary.