Confused, I need help on different fuel pump wiring methods.

[5.0 Fiend]

If I were to mount the relay in the hatch area, would 12 gauge wire be big enough to run the whole way up to the battery? I was thinking about putting it by the drivers seat, kind like a half way point, any good ideas on how to route the wiring up to there? Up through the spare tire well, then down into the kick panel? Come to think of it, I think there is a wire already back in the hatch, I think the previous owner had a sound system.