It could be that youve got a dogey earth and the previous owner wanted to wire up something like an amp or winch that draws a fair whack of current without it turning other devices off, or being turned off itself by other devices. I know that is the case for my 90, when i put a headunit in whenever i put the lights on main beam it would turn offf then after a while come back on after the surge of current needed to light the fillimants. So to sort this for now ive just connected an earth directly to the batt and it works fine.
Although if you dont have this problem, it could be that the previous owner wired in a winch or something, as i hear its reccomended to wire and earth directly to the battery.
Ollie