you have a massive short somewhere and with todays complex and plentiful wiring your chances are slim without plenty of time on your hands and test gear.
You could try first isolating each consumer in turn from the mains and reinstating the fuse link. This could take a long time and use many fuses but most garages will probably do this anyway. Look for in general any chaffed worn wiring going through bulkheads next to pipes, cables or hot areas.
Good Luck
Lee
March 2005