Where is the easiest place in the bulkhead / firewall to pass additional wires through?

ta

next to the pollen filter seems popular, under the l/h (nearside) plastic scuttle panel. Theres a grommet there already i think??

I did this on Saturday and as above this is the best place for it. Firstly remove the glove box ( held on by 5 torx screws), then remove the left hand side of the scuttle panel, the top edge is held in place by a little lip along the bottom of the wind screen. I found the easiest way of getting it started was with the thin edge of a plastic ruler. Once you got it started just pull it up vertically. Next you need to remove the pollen filter which just unclips from the holder, then remove the holder by undoing the two plastic nuts. Once out you will see a plastic grommet at the back. Remove this and it leads directly into the cabin behind where the glovebox is. HTH.