Thanks for all your answers. The differance is so as said the frequency of the PDC in the front and in the rear not the volume, so you can hear at the sound if its the rear or the front one. I've found the speaker in the rear. It was near the factory speaker of the radio. I've got two pics of this PDCspeaker. It's a very little one. There is a BMW number on it 68 21 1 379 889. I'll change the place of this speaker from the rear to the front so I can hear it allways very good.
http://home.planet.nl/~andel196/fotopdcluidspreker.jpg pic 1
http://home.planet.nl/~andel196/fotopdcluidspreker2.jpg pic 2
