If you need to install a RPM shift light such as an Autometer 5340 in a C5, I recently installed the one I had on my ZR-1.
You need to splice into pin 10 off the red PCM connector, which is a white wire, and also make sure that the shift light is setup for a four cylinder ignition to operate properly.
The issue is that the "tach output" off the computer on the '98+ LS1 motor is the
same as that of a four cylinder ignition (2 pulses per crank rev).
Making this cylinder selection adjustment has made the light work, and no ill effects should be seen.
This now gives you a way to set when the shift light comes on as to what RPM module you plug into the shift light unit.
This is great for when doing drag racing and you want to shift every time at the same RPM.