Firstly, this is on an Australian 1999 9-3 S and therefore is right-hand drive. This could be different for LHD cars.
To make it easier, and shorter, I’ve kinda just covered the main steps. Ask questions if you want more info.
STEP 1: Remove the fuse box in the engine bay and locate gromment on the firewall.
STEP 2: Cut the top off the grommet (there is 1 big hole and 2 little holes). Feed the cable through into the cabin.
It’s easier to get someone inside the car to pull it through, but not too difficult. It helped me to pull out the glovebox and the air con tubes to be able to see it easily.
STEP 3: Connect to the battery. Don’t forget to mount the fuse close to the battery.
In this case, I’ve actually ran two cables (4 gauge for the bass amps and 8 gauge for the 4 channel amp) so I have mounted two fuses. The one above is for the 8 gauge and below is for the 4 gauge above the other fuse.