Has anybody used the gray tach output wire from an msd box to hookup their tach, or do you just use a wire from the negative side of the coil? I'm hooking up the rallye dash in my car and I had the tach converted to work with an msd ignition and the label on the tach says to use the negative coil wire. Wouldn't the gray wire from the msd box be the same signal wire as the negative coil wire? I already have the msd wire behind the dash and I would rather use that if it'll work instead of having to run an additional wire through the firewall to the coil.