if you look under the glove box and pull the carpet back a bit there
is a large cable going to the bulk head and there is a grommet around it
or there is on the astra J 5 door.
other wise you may have to use the grommets in the fire wall for the heater matrix pipes
Have a look in my thread, I've done this (bitch of a job) I will upload some pics of it later. Cut the nipple off, widened it with a sanding drum and fed through from the cabin. Had to take scuttle and header tank off to get to it.