What is behind the firewall in this spot?

[Peroni]

I need to put a pair of 1/4 holes thru the firewall where the two black dots are. They are right over the brake booster. Where will I break into on the backside? I think they are too high to come into the cab. Perhaps in the wiper linkage area? Would like to know before I drill!

 

[Garbage_Mechanic]

I was looking at this as I was taking a break from resealing the oil cooler. Using a thin wire as a depth gauge through the perforations in the cowl I believe the spot weld seam just above the booster bracket is the floor of the cavity that holds the wiper arms. So I'm pretty sure you are drilling into the wiper cavity.
Can't wait to see what well thought out and neatly executed mod you have planned next!