lid alignment very important as a first resort. To help pinpoint any leak: 1) remove all trunk trims. 2) get into trunk with a good torch. 3) starting as LOW as possible with a garden hose wet slowly round the car working your way SLOWLY up. Leak will be obvious when you get to it. Then hope your helper is reliable enough to let you out again.......
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