It could be a faulty glow plug relay, inside boot light on, or a short somewhere else.
Test the glow plugs by removing the leads and check for a voltage (sparks) when they should be off.
Short elswhere, bear in mind that the fault is in something that is always on (radio, lights, etc). You should remove the fuses from these and check for a voltage when they should be off, (remove radio). If you cant check voltage then look for little sparks when you insert fuse.
It is also possible you've got a faulty alternator, it happens.
Jmmy, December 2009