I'm on the other side of the state or I'd help ya... but just go for it, its easy.
Inspect the male/female portion of plug D on your JB4 harness so you understand how it releases. Standing in front of your car looking down into your engine bay don't go straight down for plug D, use the space just to the left and get your right hand down in there (I used a small flathead to pop the clip and I did not snap it... you'll understand once you see how it works on the m/f portion of the JB4 harness). I was intimidated to start it myself... It sounds complicated, but it's not.
And as for going thru the firewall, follow the instructions on how to locate the grommet, it's easy to see when you get under there. Use a wire coathanger and tape the wire to the end and just poke it through. You wanna be somewhat careful because you don't want the grommet to pop out of place towards the engine bay; I hear thats a bitch. But all I did was kinda push the other wires aside that are already going through the grommet and that gave me just enough daylight to be able to sneak my coathanger through. I was prepared to but didn't have to remove my battery.
I watched a bunch of youtube videos on it before I went for my install... from start to finish it probably took me about 20 minutes... and I don't work on cars.