Replacement radio

Hi I have just fitted an after market radio to my 2001 scenic 1.6. I knew that the cd changer and the steering column controls wouldn't work but that doesn't matter. The main wiring ~ power and speakers plugged straight in my new radio and it works fine.

My problem is when the radio is switched on it turns the speedo off. Turn the radio off and the speedo starts working again.



Any ideas?