defender 300tdi battery drain

hi,
got a 300tdi defender and my battery only last a few hrs, have got another known good battery and that does the same
if i remove the positive lead from the battery and fit a small bulb with a wire on the terminal and the other on the lead it lights up i have pulled as many fuses/relays that i can see as possible but light stays on
removed brown wire from alternator and light is dim then connect it again and its bright does this mean its the alternator ?
any help very very welcome as i am pulling my hair out (and i did not have a lot already)

Roy

Best to check drain with an ammeter/multimeter but from what you say it looks like the alternator is causing a drain. What wattage bulb are you using?