Don't buy anything until you've ruled out the wires in the back of the radio.

I have a '95 850T that had this intermittent problem as well. If I were to hit hard bumps the left speaker would come to life. I took out the radio and everything was plugged, just fine. But, when I moved the wires that lead to the plug, the speaker would come back to life. I tied ALL the wires together, where the defective spot seemed to be, with a tie and its been fine ever since. After a few years, they can become weak at the bend.