Here is what I did. I copied this off my build thread.
So i went right above the heater core. Its a very tight fit but they all fit. I had to de pin the dbw harness connector to get all the wires through. I first put the tranny harness through. Then the engine harness. I used this rubber grommet.
2" Rubber firewall grommet . For 2 inch hole Wiring harness swap honda toyota | eBay
<p dir="ltr">2" rubber firewall grommet . For 2 inch hole. Shipped with USPS Parcel Select Ground. Wiring harness . engine swap honda nissan toyota chevy ford. or anything else you want to feed through a steel hole</p> www.ebay.com
I had to drill the hole very accurately so I wouldn't hit the package tray and so it was high enough away from heater core. Not going to lie, it was tight for the 2inch hole lol. It looks clean in my opinion but I know there are bigger grommets that people use that could make it easier. I had to slit my rubber grommet to get all the plug ends through.
I also had to extend the power wires to the battery in the trunk. They were about a foot short to route them nicely away from everything. Not a big deal but something to consider.
I also un did the harness to pull some of the wires that went inside the car so I didn't have to feed them through the firewall twice. The power tap harness was just long enough to make it back inside the car where I hooked my alternater wire and flex fuel wiring to. The tricky part about the power tap plug, is that it wires are going to the engine harness and to the ecu . So you can't get any length of it. The input/output harness can be peeled back to get length. Everything else i routed under the dash and just tucked it away from moving parts. For my drive by wire cable, I drilled a hole in the middle of the firewall. The instructions state that it should be away from "noisy" cables, so i didn't want to route that through the big hole.