Where is the best place to run a wire through the fire wall on a 1988 F 250 diesel, without drilling a new hole?

 

If it's an automatic truck, look up high near the brake booster on the pass side of the booster. There will be a small black plate covering the hole for the clutch master cylinder when the truck is equipped with a manual tranny. You can unbolt this plate, drill your hole, install your grommet(or piece if vacuum hose to act as a grommet) and re-install the plate.

If you have a manual tranny truck, I believe I was able to run some wires through the firewall over behind the parking brake lever. I can't remember what was over there, but I think I was able to poke a hole through a rubber grommet. It's a little hard to get to the wire on the firewall side, since it will be down behind the inner fender below the hood hinge, but I believe that's where I have run my trailer brake control wires and my snowplow control wires.

 

Thanks Franklin2, I was looking there but it looked pretty tight to get too. there is a bundle of wires that come through there in a white plastic fitting, with a bolt head on it looks like some kind of a cap?.it's standard shift. I will look at the site you suggested to see if that will work.