What make/model is your car? Every car has pre-made holes on the firewall. You can't miss it. They're round, 1/2" in diameter. Some are larger. They will have black rubbery plugs. You can take one of them out and fish the wire through it. Then put the plug back into the firewall. It'll keep the water out.
If you don't mind getting dirty, look under the dashboard, near the gas and brake pedels. If you look carefully, you will see some kind of hole covers. One of them will lead you to the firewall. Just lift the cover slightly and it'll reveal the plug. You can even try the same thing on the passenger side too.