Bingo if the backup alarm is running constantly that is likely your problem.  Ran into this a few times.

Solution Number: 78346

Solution Summary: Back Up Alarm Activates or Autotrac Disengages Unexpectedly When Machine is Not In Reverse - 70 & S-Series Combine / S690

Publication Date: Aug 23 2012

**Paper copies of solutions may not be the most current solutions**

Complaint or Symptom :

Backup alarm activates and/or Autotrac kicks out unexpectedly when vehicle is not in reverse.

Problem or Situation :

The electrical wires for the backup switch are present on all machines, including those that do not have backup switches installed. The backup alarm will be activated if these wires are shorted together, possibly due to damage or wear. If the backup alarm is activated on a machine with Autotrac, the Autotrac system will deactivate.

Solution :

Inspect wiring harness leading to backup switch for damage. Circuit code 904 has to reach 2.5VDC or greater for the switch to appear closed and the nearest source of positive voltage is cc812 (+12VDC).