from experience, that grommet behind the battery is the best option. Even then, it's a bucket load easier to feed the a wire in from the engine side, attache the wires to in, then pull them out. Trying to feed from the inside out is difficult. Since the kit I have is from a German site (ie LHD), I'm going to have to extend all the wires to go the whole way round.
You'll need to take the battery out which is a slight pain in the butt, and then find a good place to secure the wires to the front bumper. I'm trying to find some of that plastic wire cover for this task.
Taking out battery - note silver circular securing thingy

Further right of above mentioned silver thingy

push a wire through one of the nipples

and it pops out under the glovebox (you'll need to take out the black cardboard)
