[Johnythree]

There were a number of successful AM stereo broadcasting methods proposed and trialed. These were completely compatible with conventional AM transmissions.

The conceptually simplest of course whas where the LSB and USB are used as separate channels.

Although most of the systems did work, they were not ultimately successful simply because insufficient stereo receivers reached the market.

Go search in Wikipedia on "AM Stereo".

[bonzini]

Of the methods listed on Wikipedia only ISB is a true AM. All the others use phase modulation for the difference signal, as an easy way to achieve compatibility with mono AM receivers; and PM is basically the integral of FM.

Wikipedia says that there was basically one station doing ISB stereo; which I guess is close enough to "nobody did it", but not quite "it was never done".