I saw this on ebay... I don't know if any Sony boomboxes used a corded remote... or if this remote would work at all. All I know is that JVC used the same remotes for some of their home stereo and boomboxes. I imagine if you can use this... you know!
Sony RM-70 Remote

thats almost worth rolling the dice on to see if it might work with a m90 - the plug sure looks similar

its still a project to study each pin configuration --but might be well worth it