That grommet is the easiest & best way. There's already a debate here about drilling into the firewall. I wouldn't do it at all.
Just take the plastic shroud off around the wipers (have to remove wiper arm) and the shroud is two pieces so both wipers need to come off. Then unscrew all the screws (some covered by black plastic caps, others are torx screws under the hood. Remove that, look in that hole by the wiper arms and you'll see the grommet. It actually unscrews I believe, but you can poke a hole right in the middle, run the wire through and look by the gas pedal and there you go!
