Is there a power distribution block or additional fuse panel somewhere in the back of the van or behind the rear radio?
Yes, there is an additional fuse panel. There should be a circuit breaker under the hood wired to the positive terminal of the battery. The heavy wire from it will lead to the aux fuse panel. Check the white wires you mentioned, they may simply provide a ground (via the switch) to operate the lights.
BTW...how did that 1 ton Dodge you were working on turn out? Or maybe an update on that '93 G20?