Check your firewall for blank rubber grommets or seals. If you find a seal you can cut a hole and run the cable through there.
DO NOT DRILL A HOLE UNLESS YOU KNOW WHATS ON THE OTHER SIDE.
If you cant find an entry point (if your car has A/C) there's a sorta vent where the water drips from when the a/c is running.
You can cut a hole through there but be careful because other cables also run through there.