I'd have to dig for their contact info, but there use to be a company
called comspec that made pl encode and decode boards for commercial gear.
If you only need one tone, I'd get their encode only board, very small.
It has dip switches to set the pl and a level control. It could easily be
hid in that radio. The board has 3 wires, ground, 12v plus, and audio
out.
73s
Butch Bussen
wa0vjr