[topgun]

I personally would connect to the wires under the hood as shown in the link above. They can be tapped under the fuse box. But if you want to use the ones under the dash, look on the firewall near the brake booster. There is a large rubber boot that has the main harness going through. Near the bottom of the boot, you will see a separate small nipple. You just cut the tip off and then feed you wire though. It is originally for the backup camera wires but no reason you can't use for your purpose. They recommend adding a bit of sealant after you run your wire to close it back up. But that is what the nipple is there for.