Do you have 'flash to pass' on the turn-signal switch?
As mentioned don't overlook the lights themselves'. A test light on the plug will answer the question.
I've spent time myself chasing the dark.
Mentor asks...'have you checked the bulbs?'
'Both at the same time?'
'yes...they were both installed at the same time, they both have exactly the same usage.'
Lesson learned. First things first.
"Sticks & stones can break your bones...and hail will dent your Airstream"
So when is this..."old enough to know better" supposed to kick in?