Run a short wire from the brake/turn/tail socket to the marker light socket. If your brake /turn lights are working then their sockets are grounded and if you have 12v at the marker lights then their sockets are not grounded and the wire should ground them.
The tail light wire and marker lights wires are brown, so you may not be getting power to the tail light filaments in the brake/turn sockets because of bad sockets or those 2 bulbs might be bad as bhap has suggested, and your two marker sockets may not be grounded.
Here is a wiring diagram showing how the markers are wired and the tail lights.
