Unplug everything on that fuse line from the device (Radio, Domes,Lighter, Ect..) and with a the fuse removed, measure from the Device of the fuse (the non hot side..) to ground..
If it is infinite, the harness is not shorted. If it reads 000, you need to inspect each wire from that line for a cut or short.
If you have a bunch of 30 amp fuses, you can unplug all the devices, install the fuse, see if it does not blow..If not, plug each item back in until the fuse blows..that is the offending item..repair or disable it.
The usual offender is a shorted lighter socket..(power to ground short in or at the rear of the socket)
What year astro van is it? I'll see if I have a Schematic for it.