Given under-hood temps, stay away from obvious heat sources; you'll want your wire and main fuse easily accessible and out of the way for engine maintenance. Not sure about the grommet. I've installed a lot of stereos. I'd recommend as large a wire as the amp will handle and one that's several thousand strands... oxygen free of course. I'd locate an area clear in the firewall and clear/accessible under the dash. Buy a grommet to fit your wire. Then just drill, prime the hole, wait an hour or two and place the grommet pull, then connect the wire, and finally seal it with some silicon sealer. What kind of sub are you going with? In a box or free-air?