I know Jim at Agri Services and having seen his work, I just doubt his harness is wrong so I also have to go with somethings NOT wired right.
A while back our friend Tom 43 raised the 24 volt topic and Tx Jim there and several of us including yours truly covered it in great detail, you may wanna go back and review all that.
In order for the switch to best operate all the 12 volt POS grounded devices as well as 12 volt NEG grounded devices in cases where the two arent functioning is series (wouldnt require any frame ground connection at the batteries mid tie point) the A and B battery feeds up to it need to be in place and where the batteries tie together in the middle that point has a small jumper to the tractors frame ground, I agree with Jim, see if thats in place????????? if so then re check you have the correct harness for your tractor and the wiring just once more
Good luck, let us know what you find John T