Fixing water leaks in the trunk

The most common leaks causing trunks to fill up come from improperly sealed tail lights or body seams. Also, do not ignore tiny cracks in the lenses, water can migrate through the smallest cracks. Replace the lens, or seal cracks with clear silicone sealer.

Check for signs of water stains below the tail lights.
Replace tail light gaskets if the tail lights are removed or if signs of water leakage exist.
Check the condition of the trunk lid seal. An easy way to see if it is sealing all round is to smear grease on the top lip of the seal then shut the trunk lid. If there is gaps in the grease transferred to the trunk lid, the seal is not doing its job and should be replaced. Join in seal should be located at the latch for the trunk lid.
A common leak on pre-88 lights is where the lens is glued to the housing. Seal this seam with clear silicone sealer
Check the seams between rear quarter and tail panel for correct sealing
Another commonly overlooked point of entry for water is the license plate mounting holes. Remove the license plate, and the interior trunk trim from the tail panel. Squeeze each plastic insert and push out of their holes from the inside of the trunk. Place a small amount of clear silicone sealer over each hole, then replace the plastic inserts. Mount the license plate with all 4 screws, place a little silicone sealer on the threads of each screw.
Finally, check for correct installation of the sunroof drain tubes (green arrow), and the existence of the sealing flap (red arrow) on each side air outlet. Left side pictured

E30 M3 Repair Manual V2.7. Copyright Š1999-2001 Koala Motorsport & Brett Anderson