I got this MSD rpm windows switch to be installed on my 12v '96 (to complement my nitrous kit).

I tried to tap into the tach signal (blue/green cable) from the ECU, but if the MSD switch is turned on (12+ connected), the RPM on the instrument cluster just die showing no signal.

Does anyone have any experience or suggestion?
What kind of ignition that 12v have? inductive or capacitive?

Thanks.