Here's a link to the wiring diagrams for your car.

http://pliki.mojevolvo.pl/schematy/volvo_240__1986.pdf

The chart for the turn signals is on page 15. You may have to adapt - the wires colors seem to be different - the chart shows all green wire in the left TS circuit, and no orange wires.

All three lights on the left turn signal circuit (front, rear, dash indicator) are powered from the same post (marked L) in the turn signal control.

Unplugging the wire at the rear light seems to cure the problem - with a bulb out or missing, the remaining lights are supposed to flash rapidly when the switch is on.

Does this only happen when any other lights, e.g., head and tail lights, are on? If so, then you could have a short to positive in the tail light assembly itself.