headlights & interior light on the same switch ???

Please help - any ideas will help as i havent a clue on this one ?
The interior lights are not coming on when opening the doors but if the interior light is switched on it comes on when i switch the side / head lamps on.
If you then switch the rear interior light on it sapps power from the front ( the front one dims )
Im well confused - how can the on / off switch for the head lamps control the interior light ? my disco seems to have an alien controlling it ! oh & the clock is lit but no time showing. all fuses seem fine, recently had dash out to do heater motor but all was working perfect after refitting ( 5 times )
hope someone can shed some light
Will it ever do more than 250 miles without something breaking ?

It sounds like an earth problem (especially the rear light dimming the front one).
I've got my fingers crossed for ya, trouble free motoring is just around the corner.

Currently using a torch as i dont know where to start ! Ive had it completely appart inside so it could be bloody anywhere - Been pretty trouble free so far really m8 and tbh its really not been that bad - all been bits i can do meself & the poor old git is 20 years old after all
cheers for the advice greg i will check all earths i can find and if the worse comes to the worse i will rewire the interior lights strait from the battery through a switch and a fuse - problem solved ! Everything else is working as it did so its no biggy !