+1 with the trunk (boot) lock. It should have a rod attached to the lock at all times - mechanicial unlock. Check the owners hand book as to which way to turn the key. If the car did not come with one try e-bay. Worked for me.
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