1979 mercedes 300sd turn light problems

I have a weird problem with my mercedes 300sd turnsignals they work fine when the headlights are off . But when the head lights are on the right turn indicator light on the dash stays on an the right turn signals dont work and the left rear turn signal running light is out but works for indicating a left turn. Any Ideas of what might be wrong and how to correct it . It has got me perplexed thanks??? :confused: