I want to have an on/off indicator LED on my stealth-mounted radar detector. I just want to indicate when the 12v power supply is there or when it is not there for the radar detector (I control it with a switch).

I have a bunch of old radio shack LEDs in various colors sitting here. I'd like to use one of them.

I know it's been asked on the BBS before, now it's my turn... I don't think I can plug these LEDs directly into the 12v line, can I? What do I have to do to make them work and not burn out too quickly? Just resistors? If so, what value and how would they be wired?