The most common area for water retention on these,is in the boot area ,in the right hand wing well,which is behind the right rear wheel.This is where the battery is located.You will need to get the battery and the metal support frame removed to knock through a couple of pre-formed bungs.If you are not confident in doing this job yourself,i would leave it to a garage,as there are lots of wires and connectors attached to the battery and surrounding area.

My 2005 E46 convertible, as do many others, collects water in the spare wheel well in addition to the battery container. Various BMW Forums report entry via a failed rear light clusster seal or via the floor of the compartment into which the soft-top folds. That has an invisible drain hole under each of the roof frame mounting points and these can become blocked. They can sometimes be cleared by an airline with a small diameter tube attached and then shoved under the roof frame mounting plates. Pour a little water into that area and it should immediately appear under the car just ahead of the rear wheel. There are removable bungs under the spare wheel. Remove one to allow accumulated water to drain away. I contacted HJ because my light seals seem OK and the hood compartment drains work well, but water still gets in both to boot and under the rear seat cushion.

Any further suggestions, please.

Pavarotti