Sometimes it's the simplest things that cause us the greatest consternation. One of those simple things is the lighting on our vehicles. Vehicle marker signals (running, brake, turn, etc.) use either a socket for a single filament or a dual filament bulb. A single filament bulb will have one contact on the bottom of its base while a dual filament bulb will have two contacts.
When troubleshooting a bulb that's not working you may find that it's not simply a failed bulb that's the problem. As vehicles age, their electrical connections corrode and can eventually turn intermittent, or fail altogether. The corrosion or oxidation layer that forms over time is a non-conductive substance that can stop the flow of current across the bulb and socket contact point. Sometimes you may find the contacts are charred or thermally damaged.
Once you've swapped bulbs or confirmed that your bulb is good, it's time to look deeper. Remove the bulb from the socket and look at the contacts inside the socket.