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I'm trying to install a MSD digital rpm window switch (P/N: 8969) to control my nitrous kit. There are 4 wires that need to be connected.
RED -> +12V
BLK -> Ground
YEL -> (Normally Open) Ground of nitrous relay for solenoids
WHT -> Tach signal

I was connecting the WHT wire to engine speed pulse wire (BLU wire, pin 26 on Connector E (31 pins) of the ECU), but when I do this, my dash tach dies and the window switch shows 0 rpm.
Next, I tried the connecting the WHT wire to the crankshaft position wire (BLU wire, pin 7 on Connector A (31 Pins)). My dash tach is fine, but the window switch shows the wrong rpm. I think the rpm it shows is half of what it really is.

Redline04 used the same window switch w/ WHT connected to engine speed pulse (26-BLU-Connector E) w/ no problems. His car didn't have a factory tach, but mine (LX) does.

 
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How about the MSD dis tach driver PN 8913? would this solve the problem?

 
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There is a blue wire in front of the battery with a rubber cap on it. use this wire and you should be fine.

 
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I tried that wire. When I connect it there, my dash tach doesn't work and the window switch shows 0 rpm.

 
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are you disconnecting these wires or taping into them? you should use a wire tap and not splice the wires...

 
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I was tapping the wires.
Guy on MSD forums said I would need the dis tach adapter (8913), since one of the negative coil wires didn't work either.

 
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Ok, the Dis tach driver (8913) did the job . The window switch had to be programmed for 1 cylinder mode.
If any1 else has a problem with an rpm switch or aftermarket tach, definitely try the dis tach driver.