wiring car charger into fuse box

[Orion]

I'm setting up a car dvr and it uses a 12v car charger for power. I didn't want the wire dangling around so I'm going to hardwire it in. Originally I wanted to splice into the current AC but didn't want to do that in case i mess up or something.

So I figured I'd plug it into an unused fuse slot. I'm bad at electronics so a few questions.

I'll need one of these Small ATM FUSE TAP ADD ON DUAL CIRCUIT ADAPTER AUTO CAR AUTO TERMINALS +15A FUSE | eBay plus a 7.5amp fuse right. Current diagram shows my AC outlet uses a 7.5 amp.

Then just ground the negative to that metal beam?

What limits power from exceeding 12v and blowing up the dvr? is amps the thing that regulates that?

Anything else I should know?

[installerz]

Those devices fuck the terminal that they get inserted into. Please take it to a shop and have them solder it onto a wire that can handle the extra load. With all of the computerization ion cars today, it's better to have Canadian Tire blow your shit up for $100.00 than you doing it for free.
I guess that's why nobody has answered your question yet. Liability.

You could take it to any quality stereo shop to have it done for a nominal fee without blowing anything up.