[john hall]

OK, so there is no seat switch in play. Cut the key switch on--DO NOT CRANK THE ENGINE! Flip the PTO switch, you should hear it click/engage. Start wiggling wires until it disengages, shouldn't take but a minute or 2 to do this. If you can make it disengage by doing this, you'll know about where the problem is. Otherwise you are going to have to trace it end to end. The whole disengages when it hits a bump scenario, sounds like something big is loose and tugging on a wire. Maybe you'll get lucky and find the problem quickly. Its possible that despite your wiring harness being hacked to pieces, there is only one "bad spot".