Weird left turn signal and heater air flow issues

[68Bowtie4x4]

My 72 Chevy 4x4 will erratically flash the left turn signal dash light while I'm driving down the road. When I click the left turn signal it works normally, as well as, the right turn signal works normally too.

My pickup has the non-AC heater system, has brand new levers, cables, and heater fan switch installed. I have low and high speeds for my heater fan, defrost blows when I select defrost, and floor heat works when I select it. But it seems as I don't have very hard air flow through either defrost or floor heat.

Could it be the rheostat for the heater fan? Is there an illustrated heater check procedure to determine where the issue resides?

Thanks.