When I installed my alarm, I needed to run a pair of wires into the engine compartment and took the opportunity to tidy up the wiring from an earlier mod. I routed the wires through grommets installed in holes drilled through the firewall above the factory bulkhead connector next to the brake booster.
While I was at it, I added some extra holes/grommets in various sizes for future mods and plugged them temporarily with bolts. These have been very handy over the past couple of years and I now have wires running through 4 of the 5 holes.
Here is a picture of the grommets in my 03 Ranger which is similar to a 2nd generation Explorer under the hood. To make room for the drill motor, the underhood electrical distribution box was unbolted and moved to the side. There is a large wire bundle on the interior side that may need to be repositioned out of harm's way while drilling.