I have an after market bumper and intercoolers and the installer (previous owner) did a bit a crap job in not allowing enough room for the indicators / side lights to fit back in right. This made them touch one the intercooler rad and caused the wire to wear and short. Maybe not the same problem but most definitely look at all the indicators for a short or (like you say) corrosion. Also is there a problem with the hazard lights? (separate fuse)