Most aftermarket stereos have a line that comes from the back for muting the sound, think it will be labelled as

"att", this as well as the attention button you get on the front to mute the sound down.

On sonys i think you just have to earth it for it to take effect

(handsfree car kits make use of it i guess!), maybe this has become earthed somehow? would look to see in you stereo manual if you have this wire!

If not take it out then rewire it one step at a time to single out the problem!