Ive been out of the JD tech gig for a couple days so i might be a little fuzzy but here is my try

1. I believe they want radar because it is a much quicker response to speed changes than GPS

2. Im sure that Ag Express could make up any style harness you want and any length.  There is nothing stopping you from shortening up yourself.  Just check the section you might cut out for any diodes.

3. Some Power for the planter comes through the ISO connector.  I believe the error is saying check voltages between pin 1 (ground) and pin 3 (+12v).  When looking at it pin 3 is the top pin and pin 1 is the middle pin. I think this power source runs the clutches.