Can I hookup the power cable for my amps to the "extra terminal" under the safety cap on the battery? After looking at a few install pics, I saw that most of you did not use it. On the service manual, it looks like there is nothing connected to it and it looks like a perfect place to connect my power wire.

Can i do it?

 

I think the terminal you are talking about is after the cars main fuse. Therefore, if your amps had a problem and caused that fuse to blow your car would be left with no power and be unable to start/run.