I had that similar problem with my '99. Since the pre fl uses a dual filament light bulb the stock OEM light housing with the stock light bulb had to be used instead of the one provided with the halo pros. But since you don't have the pre-fl, you're contacts might be sticking. Read these threads (they talk about the problamatic piting that cause the blinker relay to stick in the combination control module [CCM]):
http://www.benzworld.org/forums/r17...lk-indicator-lights-staying-possible-fix.html
http://www.benzworld.org/forums/r170-slk-class/1400230-turning-light-wount-blink.html
http://www.benzworld.org/forums/r17...lk-indicator-lights-staying-possible-fix.html
http://www.benzworld.org/forums/r170-slk-class/1400230-turning-light-wount-blink.html