Don't know how much power you need to handle, but Clare makes some opto isolated versions.
**broken link removed**
Assuming this is for your LED tailight, the best way, by far, is to PWM the drive to the LED's for the low level, and steady DC for the full bright. Something like 60/40 on to off will be close for your needs. Playing with the duty cycle to remove any visiable flicker, my experience is anything faster than about 5mS or so, is not visible, except maybe under flourescent light.
Or, with a few extra parts, choose a buckpuck that already does this, and have two presets for brightness levels. As in put a trimmer pot set to the dim level on the buckpuck, and have a small signal ( PN100 , 2N3904 ) short it out when the brake light wire is energized.