I sawed my dead 87 vanagon into pieces once with a sawzall to see where
inside rust had begun. This was of concern to me as I had lost my 72 bus to
rust and wanted to prevent this from happeneing to my 90 vanagon. 

I found two places, one bad, and one not so bad. The not so bad place was the
inside of the fresh air duct in the front--the lip that comes down on the
inside of the vehicle.

The bad place was the cavity over the rear jack point, which as far as I can
tell doesn't have drainage. (oooppps, vw) To inspect this area, pull the
rubber plug out in the rear wheelwell and peer through that hole toward the
front of the vehicle. Rust here could be major terminal.