Which Avmap unit do you have? If you have the EKP IV and you have the data cable from avmap simply connect the yellow wire to the serial receive pin on the D180. If the GPS is connected to aircraft power and ground you don't need to run another ground with the serial wire. Make sure you have turned on the NMEA output in the Avmap communication settings page. The choices are Off, Direct, and Processed. I have mine set to Processed. You also need to select the NMEA sentences to be sent. The NMEA sentences required are: RMC, RMB, GGA, and either BOD or APB. If you have an HS34, run your yellow serial wire there instead of the D180.
Ken