[Rob Petrie E395]

Check the bulkhead connector on the firewall. P vans and G vans have had problems there for some time. Especially if the chassis builder mounted it in a bad place on a P van where it sees water regularly. I will be honest and tell you if I see one now days it does not get fixed with new terminals like we did it back in my dealership days. It gets a wire straight through the bulkhead connector and sealed up as best as I can. New terminals just come back with the same problem after a few years. I know its not "right" to do it that way but it works, lasts a long time and is cheap which is what my customers want on a older model stuff.

Just thought of something
You did check to make sure the Ignition switch is ok. I know it sounds dumb but when we are frustrated we sometimes forget the easy stuff looking for a bigger problem.