Drill a .5" diameter hole on the firewall next to driver side fender. There're other ways to wire battery cables, but I think this is the easiest. Connect 2 power cables to positive
The concept here is pretty straightforward, If there are no grommets in the firewall that are easily accessible then one must take matter into their (i'm hoping someone elses in your case) own hands and drill a hole.
Edit: How do these tools end up with "supercars" and don't know reading comprehension?