At some point in a car's life, the turn signal switch goes bad, and can lead to weird happenings with blinkers and brake lights.
But it's a worst case scenario.
In the meantime, pull all of your bulbs, clean all the sockets from corrosion as needed, I use contact grease in the sockets personally.
At the same time, make sure any grounding wire terminals that you can see are clean and corrosion free.
I like to undo crusty looking grounds - tread lightly, do no harm, use plenty of proper penetrating oil and needed time to loosen fasteners - clean the metal lightly, rub a layer of Moly grease on the area and reattach cable or wire.
Also look at all your bulbs to make sure nothing is shorted or open, replace as necessary with quality bulbs, not bottom of the barrel Chinese parts store junk.
On many old rigs, you see added ground jumpers from bulkhead to fenders, pickup beds to frame, etc, whatever it takes to establish a sound ground between parts, but corrosion from time and the elements will disable the connections.