bad/leaking earth

hi every1,new to all this but i was told theres some very good info on forums.
bought a 86mk1 golf cabriolet a few months back, after sorting a few problems on it im still stuck with a bad earth somewhere,the problem starts when i switch my lights on this causes the rev counter to go off also the mfa computer thing and the oil light comes on,the blue high beam light is on and goes off when you put full beam on.
could there b a fault with the ful instrument panel or am i right to persume its a bad earth?ive checked quite a few earths and cleand up any that r looking rough
any info or advice would muchly appreiated

cheers

Which earths have you checked?  Might be worth running a new earth to the instrument cluster (pin 1 or 2 - not sure).