Angier,
  I have these units in my panel (a GMX-200 instead of the MX-20) and know from experience that even if all the connections are correct, everything has to be configured properly in the setup menus to ensure communication.  If you're sure the wiring is good, that's where I'd suggest you look before tearing anything apart looking for gremlins.
  Good luck,
  Skip Slater
  N540ES