get a test light and have a friend do the key on/ key off, while you check those posts for power. you can probably locate a hot terminal like that to use.
the negative terminal will have a lot of wires attached to the same screw in the fuse box. crimp an eyelet onto your wire and add yours. or like you said, you can find a ground closer whatever you add.
Blue Sea makes some nice DC equipment that you can add if you want to put more circuits and fuses off of the hot you pick.