If you can point to a source of on-line information about the NAVSTAR CB202 radio, I can probably help you. I believe that radio is an obsolete model, and the manufacturer no longer exists or is no longer active in the recreational marine communication radio business.
You should consider updating your VHF Marine Band radio to a more modern radio that complies with the CLASS-D Digital Selective Calling specifications. Older radios often only comply with less stringent recommendations. Considering that the cost of a new CLASS-D DSC radio is barely over US-$100, investment in a new radio may be a better route to integration of your chart plotter and radio.
For information on using NMEA-0183 interconnections, see my collection of information on this topic for a great number of chart plotters and radios at
NMEA-0183 Interface Data Collectionhttp://continuouswave.com/forum/viewtopic.php?f=9&t=816Data about the GARMIN EchoMap series is given in the thread linked above link. See
http://continuouswave.com/forum/viewtopic.php?f=9&t=816#p5307for details.