Yep use a grommet (spelling??
)
These will save you a lot of time and hassel.
An amp needs a power cable from the battery and a negative cable leading to the chassis to ground. Its best to lightly sand around your grounding point then spray it will clear coat after the ground has been mounted properly to prevent rusting.
You also need a remote wire leading from the HU to the amp. This is so the amp turns on when the HU turns on. Alternatively you can run the car ACC to turn the amp on if your HU doesn't have this.
Oh and fuse your power cable as close to the battery as possible.
This saves cars from burning up.