The two red wire tach is a 6 cyl tach and won't work on a V-8.
I've been involved with and trying to find answers for this old question for over 2 years now. I't just something that has bugged me so I keep trying to understand it ....
My last attempt at this involved trying to trace where that danged red # 96 red wire COMES OUT on the engine side of the fuse block.
Finally, using a 10X jewelers loop to spot on the diagram the part of the exterior module it was in and PhotoShop to blow up and enhance the diagram, I found it! I touched it up pixel by pixel so it can now be clearly seen.
# 96 red (interior) becomes # 13F red (exterior)
See pic ...
CJ-/5.2Mag, our resident certified Jeep master mechanic explained this problem best recently .....
"....tach on CJs reads the pulses from the + side of the coil, most aftermarket and late model factory tachs read off the negative side of the coil. The hookups for the factory CJ 6 and 8 cylinder tach are under the dash, in series with the ignition feed to the coil. DO NOT hook the factory tach to the coil in the manner of an aftermarket tach, it will fry! If you haven't played with the wiring under the dash, it may still work with your EFI. The 6 cylinder tach is not calibrated correctly for a V8, so this may account for the innaccuracy found by some. The factory 4 cylinder tach works like the aftermarket tachs, getting the pulse from the coil negative, but it also won't read correctly with a 6 or 8. The exception is if you are using Mopar EFI, factory or Mopar Performance, which supplies 2 pulses per revolution (4 cylinder reading) to the tach. There is a place in Riverside, CA. United Speedometer, that can recalibrate the 4 cylinder tach to accomodate a 6 or 8 cylinder. I'm not aware of any aftermarket tach that would fit in the original dash hole perfectly."