Here is what I always do:
Remove Grille
Remove Driver's side headlight assembly.
Remove nut on antenna and thread down into body of van and out through the
front.
Cut the wire going to the old antenna and throw this antenna away.
Get the new antenna out and attach the end of the new wire to the cut end of
the old one with tape.
Go inside the van and pull the ashtray out.  You should be able to see the
rear of the radio at this point (you may need to pull the radio out a little
to get a good grip on the antenna plug).
Unplug the old antenna wire from the radio and gently pull it into the inside
of the van.  If it hangs up you may want to push it back and have someone on
the outside of the van pushing the wire as you pull on it.  
When the wire is threaded so that the new end is at the back of the radio,
untape everything and plug it in.  
Secure the antenna to the body of the van and replace the headlight and
grille.  
You are done and it should have only taken 10-15 mins.
Hope this helps.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242