First remove the parking/directional/brake & rear directional lamps from their sockets and make sure all the sockets are clean and the proper lamps are installed. That is the root cause so many times - the wrong lamp gets jammed into a socket (think a two terminal lamp in a one terminal socket)
Next pull the harness connectors off the headlights and make sure they are tight and clean too.
And the big one is, check your grounds. The parking, blinkers and side markers all share a common ground cable with the headlight on each side. The lighting group on each side has a sheet metal ground point and the RH side has an extra one that connects the sheet metal to the frame.
You didn't make any recent drastic modifications to the electrical system, did you?
