If you've got an older Viper (mine is a '96), the shift light electronics needs to support a 5-cylinder configuration. Since there are two coil packs in my car, the tach sees a pulse for every 5 RPMS (I think) instead of the expected 10 for a V10. I tapped into the cable that plugs into the back of the tach for power, ground and trigger signal.
Anyway, for the older V10 like I have, I used the shift light from VCA member Mike Brunton at
HighTechSpeed and set it to a 5-cylinder configuration. Works great!