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BMW X3 Radio Removal -- Google Search
Let me google that for you
In short, the center vents "pop" off.
Remove the hot/cold dial's cable.
There are a couple screws holding the center dash and radio in place.
Careful not to drop any screws into the dash.
The install videos from BSW, on youtube are pretty good at guiding you through this process.
BMW X3 Radio Wiring Diagram -- Google Search
Let me google that for you
You're looking for the color of the 12V switched wire & ground wires going to the back of the radio.
These links should help.
It's a really fun / easy project to do. Hope it turns out nicely for you!
...Enojoy