Have you got a Multi-meter? If not get one, so handy...
Remove the globe(s) from your interior, remove the Negative terminal from the battery then check the resistance across the terminals where the globes go. If its a short circuit, pull the fuse and relay if fitted.
It may very well be a short circuit in the wiring loom from the fuse to the lights(s). If you measure near to zero ohms with the globe(s) removed, there is probably a short in the wiring, probably near a corner. If you measure from chassis to both wires and its near zero ohms as well, measure from the lamp side of the fuse to the chassis. If it is also near zero ohms, you have proved that the power feed for the globes is short circuit to earth.