My first reaction when I noticed the front left side marker was not working while the other three were, was to change the bulb. No luck. So I tested the bulb with an independent 12-volt source: It worked. Then I measured what I get at the socket: 4.3 Volt. Same just before the socket. So, it is not the bulb nor the socket. I can't really follow the wires up as they disappear into the wrapped harness to the firewall.

Any advice on how to proceed from there?

I suspect that ~8 volt drop is occurring at a corroded pin in a connector on the other end of the harness. 

For only one of the two front side markers? All other lamps work fine.

Where do I find this connector?

You really need a copy of the wiring diagram to find out if and where that connector is. Some of the aftermarket Corvette suppliers sell just the wiring diagram by itself or you can get it in the factory service manual. I would also check the socket for any evidence of corrosion. You could physically follow that wiring harness back to see if you can determine where it terminates.

Once again...I have all of the '68-'82 Corvette wiring diagrams on my web page. Download or view on-line, either way....

'71 Corvette Wiring Diagram