[Joe Bob]

You said you had "the bulbs looked at"? A third party I assume?

The brake and running light is a dual filament bulb. The running light runs the most and in most cases burns out first. It can in "most" cases been seen visually but in my experience I've had perfect looking bulbs be bad.

Did you switch bulbs side to side? A 77 is not a spring chicken anymore.....if it's not a fuse.....try spinning it, corrosion may be a culprit.

Get a test lamp or a meter and test the socket. If you have 12v in the socket (both terminals) try a new bulb.