Batboard

Where do you go thru the firewall in a 1999 Lumina?

You can drill in the area behind the glove box. There is also a little grommet near the steering column as well.

Otherwise... there is a large grommet on the A post on the passenger and driver's side that gives access into the fender well. From there you can snake into the engine compartment. Not the best solution in the world, but it would work.