What about using a non-inverting opamp circuit? Place a cap in series with the + input to eliminate the DC component, and then amplify the noise. How would this work?
There's a forum with someone that took two wires from his ECU and wired them together, then used that new signal as the input to his tach. Every time his fan kicks on his tach dies. I told him to try that opamp circuit. Don't know what the results would be, but its got to be better (and potentially less destructive) than wiring two unknown wires together.