well i have never seen a 3mm led take straight up 12v i am pretty sure you have to have an resistor. get the smd they are really tiny. thats what varad,fusion and etc used on there led bars.where are u putting this that you dont have a room for the resistor?
Are you sure it uses 12v? If you are replacing a LED on your car with another LED. It probably uses 1.5-5v.. some incandesants use lower than 12 voltages as well. Use a voltmeter to make sure you need a 12volt source. If you are running off a carputer ps you could utilize the 5v line. What are you doing that requires 12v? Something like my tutorial?