I need to run an 8 guage power cable from my trunk to the battery in a '99 Diamante. How should I do this? Under the car? Through the interior and through the firewall? Any help would be appreciated.

 

You can put it on the driver side door.

Open the door. look at behind the hinges top and bottom. LOOK there should a small opening going to the trunk or engine compartment. use a hard wire as a guide with the electrical wiring attached or 8 gauge is think enough to slide

Just look for an opening. remember open the driver side door.

If not I don't recommend PUTTING A HOLE on your dash. Why?

Water will get inside using the wire that you put as a Bridge. it is like straw but the water flows on the wiring going to your dash and it might cause a short circuit.

Also check if there is a rubber patch or O ring that is on the firewall. anything that is an opening can help you.