How do I find ground for dash lights, radio, and running lights?

I have a 2000 Astro Van with Explorer Conversion package.  The dash lights, radio,
and rear running lights stopped working. In addition, the auto on for the headlights
stopped working. Everything else is working. I checked all the fuses and cleaned all
the grounds I could find int he engine compartment. I also checked and replaced the
rear running lights. I suspect it is a loose ground wire somewhere. Any ideas on how to
narrow it down and where to find it? Thanks for your help.