You'll want to power the auxilliary terminals/fuse block off the battery instead of the alternator, so it has a steady supply of power. If you want the new block to be switched with the ignition, you'll need to employ a relay. Each device that is wired to the new block needs to be grounded, of course. Some blocks will feature a negative terminal or bus that can be attached to ground, and then the devices supplied by the block can also be grounded there. But the block itself does not need ground. I have used a product like this or similar on several auto and motorcycle installations, and it works very well:
https://www.amazon.com/gp/product/B000THQ0CQ/?tag=joeychgo-20