I think I know this one....I had the exact same problem with my '93 T-Bird
which shared some of the same interior controls as the Rangers of that era,
I believe.   It ended up being not the dash switch, but the high beam switch
in the turn signal stalk shorting/crapping out.  The whole turn signal stalk
had/has to be replaced.

Funny, I had it in for service twice and they could find nothing because it
didn't do it for them.  After picking it up the 2nd time, I started driving
home and it did it again, so I turned around went to the dealer and parked
it facing the large plate glass window of the Sales and service department
and let it flicker and asked them if it was my imagination! ;)    Two bad
things happen with this problem - 1) obviously the dangerous driving at
night and your headlights go off and on, and 2) your driving along at dusk
and your totally unprepared when the guy in front of you sudenly slams his
brakes on because he thinks your flashing him...  The only way I could drive
at night until it was fixed was to drive the entire time with the turn
signal stalk pulled back toward the driver keeping the high beams on (which
the oncoming traffic just loved!).  You couldn't push the stalk forward as
this toggled on/off between the high & low beams and  the flashing still
occured (this is the part of the switch that went bad I was told).  You had
to pull back which allows the low & high beams to be on at the same time.
The lows would do their flashing and the highs would stay steady.

Good Luck,
Scott