Audio lowering when parking sensors not active?

[Curtly]

Hi guys, I have a bit of an issue that I wonder if you can help with?

Basically I took off my driver's side door card the other day to finish off my sound deadening (took the speaker out and disconnected a few wires). I then reconnected everything and replaced the speaker, then turned on the ignition and checked the speaker and all the electrics in the door - which are all working. Now comes the problem, everytime I drive the car off my drive the music fades as the sensors come on (as it should) and then the volume returns as I drive off - again no problem.
The issue is that after about 80 metres of driving the music the fades again, noticeably and without the parking sensors being active.

Any ideas what this could be? Would a simple reset fix this? If so How do you reset the multimedia thingy?

Cheers

 

[Curtly]

Right so after a bit of driving around I seemed to have sussed it! On the same day I did my door I also changed the reversing light to an led. What's happening is everytime I select reverse the bulb is knocking out the DAB signal (the display shows FM in brackets) I then drive off and after a short drive the Dab reconnects. The change is volume is the difference between the signals :/.