Presuming the battery terminals are fixed now - check the base of the fuse box. I don't expect you will find problems in there unless some PO has worked his 'magic'. I guess you know what to look for so I won't insult. Clock drain is minimal - like a digital watch across the battery (feel free to test). Have you quantified the drain current? You could try connecting an amp meter and checking when the drain current stops. Start that test by taking out all roof and boot light globes B4 you connect up. Start with high amp setting and work your way down. When you remove offending unit the drain current will drop.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i