Looking for some help here, I'm useless with electrics

Anyways, switch on the main lights and the dash and front headlights are fine. Walk behind and instead of the rear lights coming on, the reverse lights are on. Put the car in reverse gear and the rear left main light came on. Had a look at the bulb on right hand side and it was shot, so went and bought another. Put that in on the right hand side, doesn't work, but works on left. And the reverse lights still come on instead of main and putting the car in reverse and rear left main light comes on still and not the right.

Any ideas? Bad earth? Anyone point me in right direction of what to look for to solve?

Cheers.