Aftermarket parking sensors, using original speaker?
 

There's a speaker in the clocks that bongs when you open the door. It's a nice sound. I've installed aftermarket parking sensors in the rear bumper and they look totally OEM. The only thing that's not OEM is the sound of them, as it has a cheap tinny beep coming from the supplied speaker.

The wires that go into the supplied speaker are your generic 12v and ground. I'm wondering, how simple would it be to splice into the OEM speaker in the clocks?

Just a thought.

Seems like this would then send a signal back to the door unit when in proximity.

Depends on a few things. Does the aftermarket unit have a piezo "speaker" or voice coil?

Piezo speaker

Just realised you can buy a BMW E46 gong from its PDC for little money on Ebay. There's a few videos showing which pins produce what tones depending on which ones you wire. Looks simple enough, I'll have a go at it.

https://www.youtube.com/watch?v=pxnlJAvyr5s

Pretty much what I want. One of the pins (the one on its own at the far right) is a parking sensor beep that doesn't fade.