Best place to run wires???
 

I'm doing a VVT swap into my '90, and need to run a bunch of new wires from the engine bay to the standalone ECU in the footwell.

I ran some wires for a wideband on my previous car ('93 base), and it was pretty easy. There was a big rubber donut on the passenger firewall, and I just pierced it and ran some wires. Worked out great. I went to look for that same grommet on my latest car ('90 A package), and noticed that it was filled by the A/C lines going into the passenger compartment. Crap!

It looks like the main wiring harness goes through a big giant rubber grommet in the deepest darkest recesses of the passenger side. Also looks like a giant pain in the dik to feed wires through there.

What's the general consensus? Drill a new hole in the firewall? Or just suck it up and jam some wires violently through the bunghole?

If there are no other hole options, I'd pop one through with a step bit, clean it up and give it rim job with some RTV. Finish with a grommet/plug to seal it up.

I ran mine through the existing hole. You have to pull the hvac fan to get to it. I ran a complete engine harness through there in parallel with the factory Miata harness, but it was like trying to shove a baby back in. I also pulled the fusebox and possibly the washer fluid reservoir under the hood so I could get at it from that side.

Any pics of your finished routing?

I'll see if I can shoot a couple quick ones. I didn't take any during the install. I basically cut a slit in the rubber boot to enlarge the hole, then shoved the harness though from the engine side (due to having less harness to feed through going that direction). I then pulled what I didn't need of the factory harness back to the firewall, coiled it up, and crammed it behind the washer reservoir. Inelegant but effective.

Run them all underneath the car, along the bottom of the frame rails

If you're running a standalone, many of the wires you need should already be there. There a bunch of wires that run to the engine bay for emissions crap that you should delete anyway. Repurpose those wires. At the very least it will minimize some of the work of pulling 6 feet of wire through the whole harness.

I drilled a portal in the passenger footwell for everything. If you don't have AC, use the oil holes there.
Our 1991s only have "spare wiring" for is the charcoal-can purge valve...but I can't even remember if there is wiring for that. Even if there were wiring there, it would be way off the injector harness so it barely makes sense to use that, plus he already has grounds for other shit running through the car. On my 1995 car, there are easily 12-spare wires on the passenger side of the engine, not the case on my 1991. His 1.6 era car also has the computer in the passenger foot-well so this requires no extra work.

Believe me, if anyone obsessed over the little details like this, it's me since I'm functionally retarded...but learning quickly.