I put an Alpine CDA-9856 head unit in a Dodge today, and when it's playing the radio, all of the speakers work, but when it's playing a disc or mp3 player, only one speaker works...
Any ideas?

Something may not be compatable?

Are the speakers running off of an amp or just the head unit? I'm assuming just the head unit. Is the head unit new or used? Alpine makes some good stuff, I'd contact their customer service, they'd probably be better able to let you know if there is a wiring issue or a head unit issue.

Sal