Viper 791XV and 02 Mustang GT

I'm installing a new 791 in my 02 mustang; I had a audiovox alarm/remote start in it that i installed a while back. My problem is i can't get the 791 to find a tach signal. I'm just tapping into one of the coils, and that worked fine on the audio vox, but the 791 won't see it. I've tried changing the jumper to "low threshold" and that will make the LED flash rapidly when i go through the learn process. But it won't go on solid showing it has learned the tach rate. I know the mustang gt has separate coils for each cylinder, should I try using diodes and tapping into several coils? Or am I just missing something? I have double checked and the unit is programmed for "tach sense" not "voltage sense". Any suggestions are greatly appreciated.

I'm using the negative side of just one of the injector coils right now, just like i used on the audiovox remote starter i had.

Thanks

whats happening i believe is the tach sense is sensing an over-rev condition and shuting it down.  i believe you can perform a shutdown diagnostics on the 791xv  by holding the valet button turning the ign on then off and releasing the valet button then press and release again.  the led should flash a certain number of times.  you may find it easier to just give up and go voltage sensing.  just make sure if you do disconnect the tach wire completely. 

PS  i hate doing remote starts in those mustangs.  i had a bad experience with one that wouldn't stay away after it was installed.  still don't know what the problem was.  its the only one i gave up on.  i think i've been hexed by mustangs. blah blah blah

Thanks for all the help, finally got it working. I decided to tap into the negative side of one of the injectors itself instead of going to the coil. It works perfectly now. I was thinking about just giving up and going to voltage sense. But i can't let a machine beat me ;-) Thanks again for all the help

James