My 99 Deville had a perfect little round plastic cap that I just popped off and drilled hole for grommet and wire. Right through the firewall into the cabin, it was almost too perfect. It's located right behind the drivers side strut, just to the side of the master cylinder/brake booster. Hopefully it's the same on yours. Good Luck!! Hope you've already read about the 96-99 aftermarket hu nightmares, plan on relocating stock radio somewhere else.