Following the drop down menu's and index numbers to select correct bulb. I plumped for the Philips VisionPlus P21W (12V 21W). However the dud bulb removed had a twin element. I fitted the replacement bulb, which burns very bright when lights are on, but burns no brighter when the brake is depressed. If I switch off the lights, and depress the brake, it lights up to the same intensity as when running side lights. What is the correct bulb of equal quality I can use?