Noob Needs Help

Hello, i am new to the forums, just bought my ridgeline last weekend. So far it is a great truck and I am glad I bought it, I must say though i am sad to leave my other car behind (traded my 05 S2000). Anways here goes my little rant.

I am trying to hardwire my truck with the radar detector, i did a search and noticed that you can Hardwire it above the lights center console. i wanted to go a differnt route. I wanted to just Hardwire into the fuse box in the interior. Doing this because the switch has the lights when my Radar goes off. I want the switch down low, not in view.

My questions were; I couldnt find a **** place to ground it at, I tried to take the nut off that was bascially acting as a brace for the parking break up next to the firewall. But then i dropped the nut and fell under the carpet.

Questions.
1)Does anyone know the size of this nut?
2) which socket is the best place to plug into for the fuse?

It was easier with the S2000, it had 3 accessories fuses to plug into, and you could ground the wire almost anywhere.

This is my 2nd time doing this, so any help/advice would be greatly appreciated
thank you,
Troy.