What's the amplitude, peak to peak, of the signal? I am assuming you only need the 12V for some reference potential. It is unlikely you will be able to derive much power from it.
A couple of approaches to consider are a low-pass filter followed by a voltage control/multiplier or up the voltage to above 12V using a transformer or voltage multiplier, then rectify and filter. Do you want it referenced to the same ground as the tach signal or do you want it isolated?
John