As Dave says, it's gotta be an earth or towing electrics related problem.
Does the vehicle have a tow bar & electrics - if so, check the socket, it's probably full of mud/crud/dead spiders etc. Also trace the cable from the socket to where it joins the main harness - if someone has used 'scotchloks' to make the join, there might be a bad connection there.
Alternatively - maybe the flasher relay is about to flash its last ???
Jim