headlight help! r50 2003 cooper

Hey!

recently bought a mini and been having a whole load of troubles. i'm new to cars and don't have a massive amount of background knowledge. the other night after driving home i turned my headlights off and engine off but one of the headlights remained on, i tried turning the car back on and off but it still remained on and i ended up disconnecting it to not drain battery over night. plugged it back in in the morning and it was working fine until the same has started happening again. when the engine and lights are turned to the on position the drivers side headlight works perfect and the passenger doesn't come on, when it's turned off the passenger light comes on and the drivers side works as normal and the passenger side needs disconnected still. any help would be amazing! i think i may have a break in a cable but not sure where to start testing!