Since I got my motorhome last year (Compass AvantGarde 130), I've found it really unusual to have no sound coming from rear speakers while driving.
I found speaker connections in two top lockers and discovered that they came from a connector hidden away behind the switches and gauges inside the wardrobe along with a power supply lead from the hab battery. This is obviously set up for fitting a separate stereo/head unit into the side of the wardrobe which I know many people do.
Subsequently, I found the outlet for the cab rear speakers from the head unit tucked away in the base of the "B" pillar down beside the driver's seat (not connected) - a 6 way plug floating about with a corresponding socket a few inches away. I checked this outlet with a bit of wire and an odd speaker - bingo! - music to my ears!
The last piece of the jigsaw was also tucked away but in the lower main wardrobe behind the wiring cover - a matching female plug for the one I found before in the upper wardrobe - snipped off the cable tie that held it to the main bundle of wiring and fed it up through the shelf to mate up with its partner - wiring complete apart from fitting some speakers in the top lockers.
First photo shows the socket and connector at the base of the B pillar and the plug actually fits across the way in the lower part of the light coloured socket.
The second picture shows the distinctive configuration of the connections on the end of the loom fitted by Compass which plugs into the Peugeot socket in the cab.
The third picture shows the power and speaker leads discovered in the upper part of the wardrobe.
The fourth picture shows the connector found in the lower part of the wardrobe which was fed up through the hole in the corner of the shelf to meet the one in the upper part.
5 and 6 just show the speakers installed but not the doors rattling!