GreenWorm,
I'm not sure you have the correct wire, but the color is correct - GRAY/LIGHT BLUE.
Here are some instructions I wrote, for another application, to find the Tach signal wire.
1. The Tachometer signal wire on the Viper can be accessed from 1a) the PCM or from 1b) the Tachometer plug behind the Tach, your choice.
If accessed from the Tach, behind the dash, all connections will be contained within the driver/passenger compartment. If accessed from the PCM, the wire must be passed through the firewall to the driver/passenger compartment.
In either case, the wire is GRAY with LIGHT BLUE stripe.
1a. To find the wire at the PCM:
It is located in the Gray PCM connector (C101), Pin number C31.
The connectors can be removed by squeezing tabs on the top and bottom.
The pin numbers can be seen on the inside of the connectors if you so desire. However removing only the back cover of the connector is all that is required. This should pry off easily without removing the connector or damaging the cover.
1b. To find the wire at the Tachometer:
Remove the driver side knee panel (above the knees) to expose the
back of the Tachometer.
The blue plug at the tach contains the Gray/Light Blue wire (pin position #1), which is the tach feed from the PCM.
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NOW, the idiosyncrasy of the VIPER Ignition - It functions as a 5 cylinder car - the tach signal is 1/2 , because it fires 2 cyliinders at the same time.
This may be the reason the your light is not flashing.
Later,