Seeing as you already have a smooth DC supply, then all you need to reduce the voltage is use 2 resistors to create a "potential divider". Values can be calculated using Ohms Law. Or you can cheat and use this online calculator: Potential Divider Calculator

Not sure where you're getting your 12V supply from, but if it's from the vehicle battery, then beware that it is greater than 12V, particularly if the engine is running as you will then see around 14V+ from the alternator/charging circuit.