I can't understand why jumpstarting caused the burnup either as the parallel connection still only equals the input voltage, the current being drawn dictated by the circuit until the vdr went low. I have seen far worse damage with a vdr/fuse combination where both have been obliterated but these were due to lightning strikes
I wondered about that as well, maybe jump started with 24v? Don't know much about farm equipment except it can be a bit "industrial".
Can't help feeling it's another case of electronics gone mad. A useless tractor because some minor part of a pcb is "nit picking".