Find out from the user to the radio or other device an indicator of the current, which is necessary equipment in the most hard mode. Calculate the power dissipated in the stabilizer when operating in the linear mode, using the following formula:

PCT=(Ewha woman-VOUT)*I n, where PCT is the power dissipated in the stabilizer in units of W, Ewha woman - input voltage, 24 V, Uout - output voltage of 12 V, I n - the maximum load current in units of A.
Pick up the chip pulse or linear regulator. She must have the following parameters:

- output voltage - 12 In Rivne;
- input voltage - not less than 30 V;
- maximum load current - not less than 1.5 times the maximum current consumed by the device;
- maximum power dissipated in the regulator is at least 1.5 from the calculated values by the formula given above (switching regulators can be reduced in several times).

If you want to listen to the radio station with amplitude modulation, you can use linear regulators - switching may cause interference.
Acquiring your chosen chip, secure it on a massive heat sink. Suitable, for example, the radiator from a large processor (Pentium IV). Fan you can use from the kit radiator. Don't forget to apply between the heat sink flange of the stabilizer and the radiator, a thin layer of thermal paste. Ensure a good clamping of the flange to the heat sink.
Enable the chip according to the standard circuit. For example, if - IC type 7812 (suitable only for radios with current consumption not exceeding 2 A), put it marked to itself, with flange up, insights down. The left output will be the input (+24 V) average - total (mass), and the right output (+12V). Sexunderwate as input and output electrolytic capacitors, 1000 µf, 50 V in the correct polarity. In parallel, each of them connect a ceramic capacitor of any capacity (they are not polar). If you use the fan, connect it to the output of the stabilizer. IC switching regulators may require connection of additional elements: resistors, inductors specially designed for this conclusions.
Connect the power input of the receiver, observing proper polarity, to the output of the stabilizer, and the stabilizer through a fuse selected according to the current consumed by the radio, to the onboard network of the vehicle, also observing polarity. Check the device at work. Place it in the insulating housing with holes for cooling. Place the finished design so that it is not blowing warm air flows out of the engine, and that it could not get the spray oil, gasoline, water.