The high-pitched whine that goes up and down in pitch with engine RPMs comes from the alternator and is entering the power wire on your stereo. Or, it is being picked up in an "upstream" component like a receiver or equaliser and entering the amp through RCA leads: that requires a different type of suppressor. Re-grounding components is the first course of action. If a suppressor becomes necessary, you may have to install more than one or more than one type, depending on the your system's complexity. Use the process of elimination to determine where to locate suppressors.
Cut the power wire (usually red) belonging to the component you are treating for noise about 6 inches from the component chassis. Strip 3/4 inch of insulation off the wire on either side of the cut.
Insert the 12-volt supply end of the cut cable and the "to power" lead of the suppressor into a butt-splice connector from opposite ends. Crimp them together by collapsing the connector on the wires with the crimper.
Insert the component end of the cut cable into a butt-splice connector together with the remaining power lead from the suppressor.
Remove the component's black ground lead from its attachment point to the car and twist it together with the suppressor's black ground wire. Crimp on a starring terminal and reconnect the ground lead to its attachment point.
Unplug the male RCA line-level cables from the amplifier, equaliser or active crossover.
Plug the male RCA leads into the female inputs of the suppressor, and the male leads of the suppressor back into the amp or other component.
Ground the black ground lead of the suppressor to the same ground point as the amp or other component.
Suppressors can be purchased on the Internet or in electronic hobbyist and car audio stores. Be certain to choose and install noise suppressor(s) of appropriate capacity. A preamp-only receiver needs only a 12-amp suppressor. A large amplifier may need a 35- or 50-amp suppressor.