Been awful busy and have not been on here much; I have had experience with it. The monitor could easily be replace with a DPDT switch and a light bulb/buzzer.
On the right side of the machine behind the tire, there is a little micro switch
Edit...wasn't done; dog hit my keyboard with her nose and sent....
The micro switch has a thin metal (about 1/2 in wide and couple inches long) contact on the switch. When there is enough tension to push the door open back far enough to contact the switch, it will make the buzzer sound. The metal tab gets bent and doesn't make good contact. Hay chaff can get under and not allow switch to be activated by the thin metal (it bends/bows right over the top of the micro switch instead of contacting it).
Lastly, there is adjustment on the tension it takes to move the door outward (tension on the bale). It can be adjusted down to let a 45ish HP tractor be able to sound the alarm; the bale will not be very tight thought. It can be adjusted solid enough to snuff a 100 hp tractor without ever pushing the door out enough to activate the alarm.
There is a short 2 wire pigtail on the switch; make sure there is good contact. Also can check switch with multimeter.
Those are the first things I would check.
Hope this helps and hopefully it was better late than never.