You shouldn't use a 7812 to regulate a car battery/alternator. That regulator needs at least 12 + 3v at the input in order it to regulate 12v. If you really want to use a regulator you should go for a 7808 instead (8v version) but if all your LEDs are already set up for a 12v system you don't need a regulator. Though if you want a diode for protection you can add that to the +ve of the LEDs (make sure you connect the diode the right way).
If you want to add a diode to a regulator, add it before the reg!