Dan,

 

I've also been researching the 396 to MX20 connection.  I got updated install manuals for the MX 20 and GDL 69 from Gulf Coast Avionics.  Garmin has a policy of not releasing install manuals to users, only authorized dealers.  I'll e-mail the GDL 69 install manual and the newest MX20 manual to you directly since the LML won't let me attach 6MB files.

 

I bought the marine XM receiver, the GDL 30 (about $700) and hooked it up to the MX20.  The MX20 didn't recognize it and I returned it.

 

I know the 396 has a serial port connection but I'm not sure if the MX20 will recognize it and control the 396.  But I do know the 396 can control the SL30 comm.  Just maybe....

 

But don't expect any solid answers from the documentation.

 

Mike Easley