In the Disco the NAV unit was nothing more than a (useless) text scroll across the LED of the radio head unit. If you have the NAV there would be a module in the passenger side of the rear cargo area.

It is either a second antenna or the data link from the NAV unit in the rear.

 

My DII has weather band built in, so I'm thinking its a separate antenna for that.

 

I tried disconnecting one at a time on my radio last night - it made no obvious difference

 

After additional research it seems my system may have a diversity antenna system which uses two antennas, one in each rear side window. Why the different size connectors is beyond me. Good thoughts about Nav and weather band, but my head unit has neither of those functions.

I hooked up the standard size antenna to the Kenwood and it seems to work fine. Wrapping a small bit of cloth and a zip tie around the second wire and stuffing it inside the dash for the day the OEM unit may go back in.