I'm having big issues with my 2014 BMW 420D Convertible and my local dealers, Ocean Falmouth. My rear drivers side window wouldn't go down. Before calling the dealer I quickly googled and was horrified to see video after video of similar 4 series convertibles ..... all with gallons of water collecting within the rear panel and destroying window regulators and amps. Turns out the cause of the problem is due to a bad design where the drainage plug underneath the rear panel is blocked off by the undertray and side skirt - pinned up against it with no holes to allow any water to pass out unobstructed.
It seems the undertray etc is the same one used for the coupe which had fixed, sealed rear windows where very little, if any, water can enter the panel. However the convertible has motorised windows and as such, like any car with opening windows, some water will always find a way inside the door regardless of the condition of the seals. This would be fine for the convertible if the undertray was changed by design to allow this dirty water to freely flow out.... but it isn't.... so water is soon trapped as soon as the smallest amount of dirty water gathers inside the panel.
BMW know all about this. They even have a service bulletin that describes the problem and the easy fix.... drilling a hole too allow the water out of the drain plug. See service bulletin B513116 .
To check, I carefully unattached the undertray, and whilst videoing I pulled out my drain plug..... And gallons of rusty coloured water flowed out.
The dealership have now taken a look for £125 too diagnose the issue even though we knew the cause, plus charged another large sum as they couldn't lower the roof - (although the roof lowered OK for me just before I took it in as I cleaned and lubricated the seals with Gummipflege as I've always done) and they have agreed the window regulator has totally rusted out. Who knows what condition the inside of the panel is in.
I bought the car from Ocean BMW, and have bought extended warranty every year since the main warranty ran out. The car has been serviced and maintained by Ocean BMW since new. I'm told the extended warranty won't cover it as the window regulator is damaged by water. And BMW HQ have refused to help out despite being told that I'm aware the water damage was caused by the drain blocked by the bad design of the undertray. The dealer say they are only a franchise and can do no more.
Needless to say, I'm not happy.
I would like to hear from anyone who has had similar issues with the blocked drain.. and whether it was fixed under warranty, extended warranty, BMW goodwill.... or, like they're telling me, passed on as tough luck on the customer.