First of all, there is a device/ckt called the Voltage Regulator alreday on yor car.
It's part of the famous Alternator.
That's a gross way of keeping the voltage from going over about 14.4 volts.
Then there are the various devices that need electricity.
It makes no sense to FURTHER regulate the Fan motors, Wipers, lights, etc.
Who careswhether a light sees 14.2 or 14.4?
BUT there are devices that need a well regulated voltage.
Things like the ECU and the Transmission control module, etc.
I will Guarantee you, that mounted on the input of these devices IS a "voltage Stabilizer". It will be part of, for instance, the ECU.
Probabaly consisting of some heavy duty Zener Diodes and output the exact voltage needed for the device. That could be as low as 5 Volts.
And that 5 volts will be there, steady as arock even if the input voltage varies from 12-14 volts.
And then there is the radio and the CD that both have regulated voltage in their "Power supplies".....
and lets not forget the Navigation stuff.....
The point is all these critical devices have their own individual "Voltage Stabilizers".
And they may "stabilize" the voltage at different levels for different devices.
So all this device CAN do (Since it is connected directly across the Battery) is attempt to regulate the Primary Voltage to a more exact 14.4 volts.
And it may have some circuitry that does that.
That is just not necessary.
Of course, it would void your warranty immediatly BTW.