Lights problem on 300 tdi

Hi

The lights on the back of my 300 tdi are not working properly. On the left side the lights work when they are switched on, but the brake light does not work. And on the right side the lights dont work but the brake lights do but when you use the brake the lights on the left side go out.

Any thoughts would be greatly appreciated

Thanks in advance

bad earth?

The wires chafing on the chassis at the back would be my first place to look

Could also be bad earths within the lights themselves. Take the lenses off the faulty ones and undo the bottom left of the three screws ... this should be the one that has the brass earth plate on. Check for corrosion and spray with WD40. When screwing back together do it with the lights switched on and see if they go on and off at all.