Trouble with replacing Audience with Stream. NO sound

Hi

I am having a bit of problem with the task of changing HU on my Octavia 06. Originally there is a Visteon Audience installed in my car and I thought of replacing it with the Stream instead.

After connecting the new HU and trying out the radio and CD I realized that there is no sound. I got worried and put back the old HU and the sound was back. I did a bit more testing with the new HU trying to get it to work with no results. So I packed it in for the day and was about to park the car when I heard the sound from the backing sensors. This got me thinking is there something I missed?

What I have seen from wiring diagrams and such, is that the sound signals from the backing sensors are connected via the HU. 

I have heard something about problems with the CAN-BUS?

any help would be fantastic.

P.S what does the Rad on+ pin do in the back of the Audience HU?