VDO 3-3/8" (85mm) OceanLink GPS Speedometer 0-14 - Black Dial and Bezel - A2C1351970001

I've purchased two. First one would not power on when connected to Garmin NMEA 2000 backbone. Returned for refund. Purchased a second unit, same result. Backbone verified properly connected and transmitting wifi via sea device transmitter. I'm sure I must be missing something. It's a silly simple hookup. Drop cable to backbone. Does the Engine analog connection have to be used? All I want is the GPS Speed function. Otherwise, I think this looks like a quality piece of equipment. I will update stars rating if I can get the unit to power up.