This problem started after your new head unit was installed right?
Well your installers might have spliced into the dimmer. (So that the radio will dim as your dash dims)
Try to adjust the dimmer on the dash, then see if your new head unithas a dimmer adjust option, if neither of those work get that head unit detached from your dimmer.
Btw. This is not an exact science, could just be a fuse, but just remember this as a backup. I've seen this problem on allot of other model cars. So cant hurt to throw in my 2 cents. Best of luck.
