when i switch lights on full loose wiper speed plus fuel gauge drops back to zero and drivers side indicators glow orange front and back any help would be greatly appreciated

hi folks i have a defender 90 300 tdi county with an electrical problem which is when headlights are switched on fuel guage falls back to zero also if wipers are used with headlights on they slow down nearly to a standstill hopefully some one can point me in right direction many thanks Tam