how to dim LED strips in passenger cars??

Well, you can wire in a resistor in series with the input power, it's the simple way.  As for the size, I'd try several values in the 22 to 50 ohm range for 12 LED's, and I'd also make them a lot larger, use a 1 watt resistor.  You'll have to adjust them to the intensity you like, so there's no concrete answer.

I use the bare strips and a constant current source for lighting them, then it's easy to adjust the intensity.

Gunrunner, Thanks for the reply and info.  Is the constant current source you use something you buy or, my guess, something you yourself make? If the latter, is it something a electronic moron such as myself could put together if told what parts to buy? Thanks , Fred

The ones I have are not AC-DC compatible and rated for 12VDC in. Mine are wired in series of 3 and have a + and - terminal. If yours are like these I would install a bridge rectifier in series to them. They will light on AC but the reverse voltage may shorten their life.

You could dim them with an additional resistor in series,experimenting with values until you get the desired results. Get some 240 ohm half watt ones,and put 2 in parallel,if it is too bright add a third,then a forth etc, until you get desired brightness.

If you run conventional this will vary in brightness. To overcome this you could install a 7812 regulator circuit,or a 7809 regulator circuit or an adjustable regulator until you get the desired intensity.

Dale H

Here's a typical circuit I use.  You can adjust the resistor for the intensity desired, and the value of the capacitor to increase flicker resistance.

Thank you all for a lot of good information. It may take me a while to sort all this out, but it is VERY helpful. Thanks, Fred

The problem I have with controlling the voltage is it's much more sensitive to changes in the voltage level.  A constant current source is.. well constant.  I agree that when you get the voltage proper it'll work, but the difference in a light strip here from 7 volts the LED's are dark, at 8 volts they're on at a reasonable low level, and at 9 volts they're too bright already.