Best way is to make a new hole.
First, go to the parts store and get a grommet that will seal for the cable size and is large enough for the end of the cable to pass thru, if it has a pre-installed end.
Second, see how much you can bend the cable without damaging it.
Connect the cable to the back of the radio and run it where it will bend and exit thru the firewall, that's where you will want the hole.
Third, drill a small pilot hole, then check both sides of the firewall for obstructions, for the hole itself but also for the cable passing thru and out to the underside of the truck.
Drill the full size hole for the grommet you have already gotten.