I had a TS, but not a V6, so I'm unsure if the hole is in use for you. But on the left side of the car (passenger), there should be a grommet in the firewall. First, remove the plastic windscreen wiper trim piece, and you'll find it under there; pretty big one, if it's unused. That goes to the area behind the glovebox; however, on the other side of the firewall is rubber-coated fibrous sound deadener; you can simply cut through that with a hobby knife.
Accessing the area behind the glovebox from inside the cabin requires a very flexible neck! Try and lie on your back in the passenger footwell, and look up with a torch behind the glovebox (there is probably a felt/fibrous cover over the fan blower motor in the footwell that you remove first).
It's fiddly, but you can feed the wire through from the engine bay and direct it down to the footwell. I fitted 0awg powerwire through the 156, so thinner wire will be easier still.