Electrical issues with my ’89 GTC coupe.

Intermittently, no turn signals but all brake lights work - as long as turn signals aren’t turned on.
When turn signal is set to left or right, only the brake light for the side signal light is set to
will light (but still no turn signal).

Recommendations for where to look? Inside steering column? under dash, left side kick panel,
grounds or ??????

I have factory manuals (mechanical & electrical) and preliminary check shows nothing loose or
unplugged.

TIA for any thoughts or suggestions.