I would:
1. Follow the steps for unbolting the antenna from the fender.
2.Lift up on the antenna to provide access to the cable underneath it.
3.Snip the antenna cable at the point it attaches to the antenna.
4.Tape a 12ft line (I've used 100lb braided fishing line) to the antenna cord.
5.Go into the cab and pull your antenna cable out from the dash.
6.Return to your fender and securely attach your new antenna cable to your line.
Make sure the line is attached and taped to the cable inline as there is likely a grommet it will need to feed through. Having the line attached to the side could cause it not to feed through such points
7.Now you can insert your antenna cable into the fender hole and return to the inside of your vehicle to fish the cable back under your dash by pulling on the string its attached to.
