I use a simple and easily reversible method.

Assuming you already have an aux battery installed you should only need
to run a single wire from it up to the dash fuse/relay panel area. The
new wire should have a fuse in it at the aux battery end. I would use a
15 - 20 amp fuse and a 12 or 10 gauge wire.

For stock vans the interior lights, cig socket, and radio should all be
running from fuse #3 of the main fuse/relay panel. First, make sure all
those items are working to begin with. Then pull fuse #3 out, counting
from the left end. Now check again to make sure that all the items you
just checked have stopped working after fuse #3 was pulled.

The fuse has 2 legs that plug into 2 socket holes. One of the socket
holes connects to all of the items that stopped working after you pulled
the fuse. If you plug the new wire into that socket hole then the new
wire will now power all of those things from the aux battery. To plug
the new wire in I crimp a male 1/4" spade onto the end of the new wire.

There are 2 versions of the 86-91 fuse panel. The fuses are in the same
spots but for #3 they switched which leg hole has a connection to all
the accessories. That means you need to test the 2 holes to see which
has power from the starting battery and then use the other hole to plug
you aux battery wire spade into.

I also like to put a label on the end of the wire telling where it plugs
in so if it gets pulled out sometime later you can tell where to put it
back. This is also useful if the aux battery should be dead or need to
be removed since pulling the wire out and putting a fuse back into #3
puts everything right back on the main battery like it was.

Mark