The Honda harness needs to make its way into the cabin. I originally thought that I would use the existing hole that the Mazda harness goes through, but I have since realized that even after I remove every unneeded wire from the Mazda harness (a future project), I still will need another hole.
I picked my spot, put some painters tape over it to prevent paint chipping, grabbed a hole saw, and started drilling.
Unfortunately I didn’t realize immediately that the hole saw I grabbed was made for wood. It made it about halfway through before its teeth were worthless. A quick trip to the hardware store for a hole saw rated for metal, and I was in. The Honda harness came with a nice big grommet and mounting ring already on it. I put it in temporarily to drill the mounting holes, and then pulled it back off. Next I added some black touch-up paint to the bare metal edges to prevent rusting. Finally, it was permanently mounted.