Another source is water can leak through the tail light gaskets. Usually simply removing the tail lights, clean the back of the lights and the mating surface of the car, and carefully reinstall and snug up the mounting nuts, and all is good. In rare cases, you might have to replace the gaskets. One has to be careful not to overtighten the nuts, being set in plastic, you can break the studs out of the light.
540 M-Sport package (Canadian market), factory Bluetooth phone integration, factory M-Audio 10" dual voice coil subs, Homelink, Eibach front and rear anti-roll bars, M5 rear traction rods, H&R 15mm wheel spacers, ST X (KW) coilovers, Strong Strut front upper stress bar, BMP strut tower reinforcement kit, Stop Tech ss brake lines, Akebono Euro Ceramic brake pads, Zimmerman rotors, Dinan Stage 1 software, USA Spec Ipod/Iphone integration.