1991 Chevy S-10 electrical noise

My high mileage 2.5 liter S-10 has developed a crackling/buzzing electrical sound that is heard through the stereo. The noise is much louder during deceleration and almost completely goes away when accelerating. There is no noise at idle. The noise does not appear to change in frequency but only in volume. When decelerating the volt meter on the dash drops to 12 volts and the dash lights dim. It does not matter if the stereo is on or not, the problem still persists.
The more accessories (wipers, signals, fan) That are turned on the worse the problem. The head lights are not, and do not affected by the problem.
Around the same time that the noise began the speedometer failed, so I replaced the instrument cluster with a used one and the noise still persisted
I have temporarily replaced the alternator and the computer(number and letter codes were matched) with used ones. This did not fix the problem.
I have checked the voltage at the battery and there is no loss of voltage during deceleration. The voltage at the fuse panel is also fine.
The stereo is aftermarket and was professionally installed 160000 miles ago. I have disconnected all power to the stereo and it did not change anything.
Several people have been baffled by this problem, I hope you can help.

Rob M.