See my Cave, LED Marker Light
First section shows How front markers work.
Front bulbs won't blink w/ rear bulbs have problems w/ thermal blinker can because thermal flasher units are picky about # of bulbs. In Fiero, Turn can expect ~ 3 bulbs but Hazard flasher expect all bulbs at same time.
Aftermarket Thermal flashers aren't made exact same as OE ones (Even if made by same company) and many are electronic units that often blink w/ only 1 or 2 bulbs loading them.
Rear markers turn on w/ taillights only and ground as all the rest rear lights.
Ground under rear "fender" is likely good because other side works.
@ sockets, check volts to frame directly then to socket ground. Should be close to same volts.
Socket see near 0v then Peirce wire(s) w/ a needle near C500 and test volts to frame there. Coat wires w/ silicon or brake grease to seal out water after.
If sockets get "12v" but bulbs don't light or both filament light when turn light on the ground part of socket is likely bent and not making contact w/ bulb side.