Is the wire still going through the door post down to the driver's side? If so, then unplug the old antenna wire from the back of the radio. Unscrew the antenna mast on the roof. Pull mast out slowly until the plastic housing has cleared the roof. Cut the black antenna wire but leave it in the hole and don't let it fall into the hole. Use strong tape (I use black electrical tape) and secure the end of the new antenna (plug) to the old wire. Gently lay the new antenna on the roof. Now, reach down and gently pull the old antenna wire down through the door post as you feed the wire down the hole with the other hand. Secure the screws in the mast, remove old antenna wire and route new wire to the back of the radio.
If no antenna wire exists, you will need to "fish" some bailing wire through the door post hole and follow the same procedure.