Unfortunately, there isn't a way to always show the marker numbers on the screen on your C-Map unit. Many customers complained that the screen was too cluttered with information, so the engineers removed them. However, the information can be displayed by putting the cursor on the marker and pressing enter to get more info. Typically heading originates from the autopilot's heading sensor. If the autopilot's information is off, it'll need to be corrected at the heading sensor/autopilot. I'm not sure what heading sensor you are using or how it's connected to the RDP139. It should be coming in on data3 only and HDG or HDT sentences should not be turned on in any port's "select sntnc" area. I'd recommend finding out which compass has the correct reading and re-swing the offending sensor.