Right signal triggers flashers

I have an '89 Graduate with a small problem: When I turn on the right turn signal, BOTH rear directionals blink. The right FRONT directional works fine.

Both front and rear signals work properly when I turn the left signal on. Anyone else have this problem? Find a solution? Thanks VERY much.

Welcome to AO.

This is a classic symptom of a bad earth (and in this case, for the right rear).

Bad earth?

Earth = ground