Using Garmin GPS 17HVS NMEA with RL70C

[Tigray]

I have an RL70C that also does chartplotting. It was set up on the boat so that it gets its position info from the Northstar GPS - Northstar outputs NMEA to autopilot, then to RL70 via Seatalk (I). That part works just fine. The boat also has a Raystar 112LP Seatalk version which was intended to give the RL70C GPS data if the main GPS was out. The 112LP hasn't worked since 2003.
So the first question is: Can the 112LP be repaired? I have replaced the lithium battery, still doesn't work.
The second question is: Can I use a Garmin GPS 17HVS to provide NMEA GPS data directly to the NMEA input on the RL70? I got a GPS 17HVS for free, and I'd like to replace the 112LP with it if the 112LP can't be resurrected.

I tried hooking up the GPS 17 to the radar; GPS 17 NMEA out to RL70 NMEA in+ and RL70 NMEA in- to ground. The GPS 17 output swings roughly +5V to -5V and runs at 4800 baud. Doesn't work. There is no separate NMEA out- coming from the GPS 17. Is +/-5V enough swing, do I hook it up some other way, got any ideas?
Thanks!