Is your NAVDATA NMEA0183 compatible? Raymarine use their own version of data, SeaTalk, which is not directly compatible with NMEA but there are converters available.
NMEA sentences, I usually select:
APA
APB
RMB
VTG
XTE
BOD
These usually work with auto pilots.
Connections are usually the problem and if still no good, you should refer to your 708A manual for the correct NMEA out connections. One other possibility, if you are sharing the NMEA output with anything else you may be dragging the data signal down. Only one device should be connected to your 708A NMEA output – use an NMEA distribution box if this is the problem.
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