Firewall Hole on Prostar

[Waterloo]

On the passenger side under the hood, where the AC system is located. You can drill a hole in the firewall near where the dryer sits, just below where the radio antenna cable exits the interior of the cab. You take the kick panel off and you should be able to easily get your hand in there. I have quite a few wires through that spot without issue. Just remove the kick panel prior to drilling, there are other wires in there. You can pretty much judge where it will be best to drill.

This is what I used, drill a pilot hole and go slow.
https://amzn.to/2Qw76gB

You can get rubber grommets that will fit the hole you drill if you take your time.
https://amzn.to/3gmZw2d