Car radio will not shut off... subs installed HELP please?

I have an 03 mercury sable. I had an amp and sub installed about a year ago. Everything was working just fine until recently. Now when i shut my car off the radio and internal clock will not shut off. If i remove the fuse between the battery and the amp everything is fine, but as soon as i put it back in the problem starts again. any input is appreciated, Thanks in advance