4pyros, re your question. One of us is missing something but at the risk of insulting you - or me making a gaff - i'll say that in this instance I'm dealing with ready made led light strips made up as 10 paralleled groups of three LEDs in series - so lets say for arguments sake, 6v for the LEDs and a recommended optimal current of 18ma. The built in series resistor(s) is presumably calculated to drop 6v at 18ma at the "rated" operating voltage for the string which is 12v. If the supply voltage is higher, the current will increase and stress the LEDS causing premature failure. The range of voltage that can be expected in a 12v vehicle will range from 12.6v (fully charged battery at rest) and 13.5v - 14.4v when the alternator is charging. Given that in my original post I mentioned using a low dropout voltage regulator, and there has been some input already about stabilising the current and that that would be impractical in this case for a couple of reasons. Come back if I've missed something...