Okay, well you got your power supply already (the battery) Now all you have to do is determine what kind of LED you want in it, if you want chasers (Knight rider lights) it'll be a bit more difficult, then you have to figure out how much power your car's battery puts out.
Then once you know what the voltage is of the battery, and you have an LED picked out, you have to get your resistor for the LED(s) You figure this out by finding out the voltage the LED can handle to get maximum illumination, what the draw of the LED is is (in miliamps, usually between 5 and 25), and what the voltage of the batter is, and calculate this equation.
R= (Vs-Vl)/I
R = Resistor Value in KiloOhms
Vs = supply voltage (Your Battery Voltage)
Vl = LED voltage (The Max Voltage of Your LED)
I = LED current (Miliamps)
EXAMPLE: So say your car is running a 12v car battery (average) and your maximum load of the LED is 4v, and it has a 20 mA draw, then your resistor value would be .4KO, or 400 Ohms.
After that you need a switch, and you need to find available space on your dash.
Wire up your circuit, and use your battery in place of the schematic battery. Now in your circuit before you place the resistor, and LED, run the positive (and possibly the negative depending on what type of switch you choose) to a hole drilled in the dash. Then place the LED wherever you want it, making sure your resistor comes BEFORE the LED.
If you plan on making a chaser LED, then I'll get you a link, unfortunately it's a 9v, so if you do have a 12v battery, which is normal, you'll need to get a 12v to 9v transformer or something similar.
https://www.aaroncake.net/circuits/chaser.htm
Got the idea ?