grounds, grounds, grounds.
the next place i would look, is at the battery cables themselves. the newer vehicles have really cheaped out on cable ends, and "green death" is commonly found, starting where the cable is terminated to the end.
if any "green death" is found on either cable, trace it to see how far up the cable it goes. you will most likely find the cable[s] need replaced.
another place to examine closely on these minivans, is the fuse and relay box by the battery.
as the battery outgasses, many times the gasses start to corrode the connections in that box, as well as any wiring around the battery itself.
chasing wiring issues can be challenging to say the least. [right now, i am trying to figure out why the door locks on my "bus" won't lock or unlock using the door switches, yet the 'rolling lock' feature works. i am getting balder by the second chasing this issue, even using the factory diagnostic manual. ]
please keep us informed as to how this progresses, and what you find.