Grounding amp to an aux battery

I'll be installing an amp in the back of my Prius.   It has an aux battery in the back to provide 12v electronics (not at all the battery that is part of the hybrid system that powers the engines)

Answers to inquires I've made regarding where to ground the amp in the back include:

1.  Grounding the amp directly to the aux battery's ground terminal.

2.  Ground the amp at the same bolt the factory wire connected to the aux's battery ground terminal is connected to.

Are those ok ways to ground my amp?

Also, is it ok to use the fuse that came with my amp kit, even though it is a 50 and my amp takes a 40?   Thanks.

1. yes provided this is the battery that is providing current to the electrical system and thus it should be grounded to the vehicle chassis. I imagine that this is also the battery that your power wire is hooked to, correct?

2. yes

3. yes