Is it stuck on steady burn, or is it flashing?. The most likely area of fault is the hazard switch. Next check if a trailer socket has been fitted, flasher wire may be bridged to tail. The third area is the actual flasher switch, but while they can give problems, it is usual failure to work, as opposed to staying on. If the van is driven by smokers, often ciggy ash finds its way on to the flasher switch contacts, and is corrosive, it damages the contacts a lot more than dust.