I had the same problem trying to get RMB & RMC to run AP.
Mine (TMQ AP4) also requires APA or APB.
So I sat down and bastardized OPEN2.5.0 to suit my needs. (Windoz XP OS)
I really think this would be better served as a plugin as my less than handywork cannot be used on any upgraded versions of OPEN.(time has not allowed me to pursue this avenue)
The reason for my reply is to let you know that while connected to the RMB/C sentences nothing was happening yet as soon as I rejigered OPEN to output A&B sentences the AP drove the boat perfectly using the very same physical connection.
Whilst I was at it I also allowed BWC GLL VTG to output as well, these all give my radar
navigational status, however bearing and distances need leading 0's as 39.0 NM is seen as 390 NM and 27.1 deg is seen as 271deg.(still more time needed)
Hence you can see what I did is far from being reliable enough to put out to the public.
But it works for me.
Thanks to all who have input to this wonderful program, I appreciate everyones time is precious.