Need help! Left Signal.

With a mechanical TS flasher, if a side is stayin on constant, it suggests that there is not enough draw on that side of the car (the current heats and cools a bi-metallic strip in the flasher, which is what makes it flash).

Ensure that all marker/flashing lights come on with the hazzards. Ensure all bulbs are of the correct wattage (in the first paragraph, an example of a light that might not draw enough current is an LED bulb, which draw almost nothing). An electronic flasher can help your situation if you have custom bulbs and the flasher is not rated for the current anymore.

Good luck.

Try calling your local parts store. In the old days, turn signal part numbers related to the number of bulbs needing to be powered (so the flash would be pulsed properly). WIth electronic flashers, that's not a concern. Just a couple of variants of E-flashers are out there - it mainly just needs to plug into the connector.
IIRC, Tridon and the Bussman folks make them (for two). They're about 10 bucks and a decent upgrade. (As I recall, 94+ stangs have electronic flashers, for instance).

Good luck.