AFAIK the only relevant resistor would be one needed by an LED lamp, there's nothing particularly related to the usage or any change of usage. (Indicators are different.) Some LEDs for car use come with the appropriate resistor, either as part of the assembly, or for you to fit. If they don't then the relevant information should be included, and there are some clever people here who can work out what's needed. The resistors are widely available. So when you've decided what to fit, and if it's an LED, post the information here. Jonathan