I recently instaled a sony head unit an amp into my saxo plus a pair of front door speakers. (6x9 already present) But when i turn my engine on i get interfence with both the radio an the cd player its driving me mad can anyone help please ?

 

Can you describe the interference? A whining noise that gets higher pitched when the engine RPM goes up?

I am betting that it is a bad connection. Check the ground connections on the amp. Make sure it has a solid connection to bare metal. Did you tune the amp correctly? Sometimes having the gains too high will amplify any interference you get also.