Go to the Garmin's interface page and select an input/output of none/NMEA. If you have a choice of NMEA output format, select NMEA 0183 2.0.
If you have a choice of output connection protocol, select:
4800 baud
8 bits
Parity = none
1 stop bit
Make sure it's connected to Comms Port 1 or 2 - Autoroute is designed not to recognise any device on Comm3 or Comm4.
I know the following is not written for the Garmin, but trying the various suggestions should enable you to see if your GPS is sending data to your PC. Let us know the results.
Microsoft