Amp

If you do a archived searh for firewall under audio & electrical you will find the thread you need. It's above the brake pedal about 6 inches up.

on mine i drilled about 3 or 4 inches about the hood pop latch. works well and since then i have done it to 2 other cougars and have had no problems the hole will come out of a thin part of the firewall and is nice and hidden. i have the same setup and it works great. hope this helps good luck

pop up the hood and look at the driver side in the front firewall about 12-14 inches in and about 7-9 inches down you will find a rubber cover. if you pop that rubber out it will give you a big hole about 1 1/2 inches, cut a small hole in the middle of the rubber and use that hole to put your cable through. that how i got on my cat

Jimmii

You said you have a MTX, I know that you can also run your wires through the brake pebal, where it meets the firewall, there's a rubber boot there that surrounds the break shaft, you can move that and fit the wire through then replace the rubber cover, and it doesn't interfear with the braking at all!
Hope that may help too(saves drilling)