Is there voltage on the wire?
If not, check 'continuity' from the wire to 'ground'... is there 'continuity'? If 'yes', the wire is connected to a device in the AS... somewhere, OR, it is 'shorted' to ground...
If you disconnect ALL power... then connect a small AA battery to the wire, you can connect the other end of the battery to a longer wire to extend to another connection in the trailer to test for 'continuity' with a tester or VOM.
Do NOT connect your 12 VDC battery or converter to this wire ... if unfused and it shorts out, you will find where it went.... just follow the magic smoke..
Peace and Blessings..