(repost in case editing my previous post doesn't count as an update)
A simple circuit for your NMEA-RS232 could be simply an Opto-isolator, with a 1K resistor in series with the input/NMEA side (the Multi itself uses 4 2.2k in parallel in series with a PC357 opto, say 560 ohms) .
The opto collector goes to +12 (with another 1k or so in series for good measure) and the opto emitter goes to RS232 in. You might need resistor to ground (a 10k or so) on RS232, but usually switching from open circuit to +12 would work even without a negative source.
I hope this verbal schematic makes sense, if so it should only cost about $5-10. Good luck (from a middle-aged (I wish) EE).
1998 Jeanneau SO45.2