You're just looking for continuity of the circuit. The positive side of the battery is connected via the main fuse , the harness and switch gear to the various electrical application on the vehicle. The negative side is connected at various points to the frame for the return run. This is the earth on a DC system. If the return (earth) is interrupted at any point it will take the path of least resistance, which can be through another application. This can be seen when the tail light flashes dimly with a dim indicator when the indicator is operated. Positive (power) is getting through but can't find it's way back and is having to come the long way. Clean all the contacts in the bulb housing and open, clean and reconnect all connectors. Put your meter on low resistance or continuity and check for a route home. In your tail light, the bulb has two filaments, one for tail light and one for brake light. They use a common return(earth) hence the three wires. The earth wire goes from the bulb to the housing or the frame. The return(earth) fault would appear to be on the indicators. You don't say if it is one or both. I'd be looking at the connectors, first.