My left blinker doesn't work, just lights but doesn't blink. Right works fine, so the flasher is good.

All bulbs are in good condition (working) except the rear side marker. Replaced the bulb but still it won't light.

Any help is appreciated. Wiring is NOT my forte ...

 

OK Steve, wiring IS my forte. Let's start with a question or 2. Is your flasher can stock or of the stock, standard type? Are ALL of your bulbs the original incandescent type?
Oh, and a rear side marker has nothing to do with your turn signals.

 

Everything is stock. A few weeks ago the right blinker stopped working. I put a new bulb in the taillight and all was good again. Then a day or 2 later the left blinker went out. Tried the taillight bulb but that wasn't the problem this time. Left tail light seems to not be as bright as the right.

Any help is appreciated!

 

Ok, Steve. First, a flasher can is basically a Bi-metal spring. Current flows through the Bi-metal spring and heats it up. The spring expands from the heat and changes shape, breaking the circuit. (The lights go out). The spring quickly cools as now no current is flowing through. Then it pop's back to it's original shape and reconnects. (The lights come back on). And this is the click, click, click, click you hear coming from the flasher can.
If not enough bulbs are on the circuit. As in not ALL the bulbs are lighting up. There won't be enough heat in the Bi-metal spring for it to click off.
So, LED's that draw very little current play hell here. And if not all the bulbs are lighting up the flasher won't flash.
Turn on left signal. The front light in the grill should light up. The front side marker should go out or flash depending on light switch position. The rear bulb in the tail light should go bright.
Check and double check the grounds on all these lights! You say a rear side marker isn't working. check that ground! If ALL lights light up and go bright but still don't flash. And indicator in dash is lighting up. Replace that flasher can anyway! The one behind the map pocket. (RH dash side)

 

Thanks 4-vettes helping my son today and tomorrow so I'll try your suggestions next week.

This one is my run about. I only have 2 vettes, the vert is getting a lot of new stuff and I hope to be driving it soon. My other cars are a 66 Mustang and an OJ Bronco.

 

As said above 'check the socket grounds'.
Connect a wire to a good ground and touch the other end to the brass part, If it works then the ground inside the socket is bad and replace the socket.