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Greetings to everybody, A Few Matters for Consultation!
Whenever there is heavy rainfall, my boot area is allowing water ingress, but I am unable to identify the source.
All the surrounding seals are intact; nonetheless, water is infiltrating, particularly in the spare wheel well.
Can someone provide guidance on this issue? Someone has proposed that there may be a concealed vent below the rear bumper, similar to many Volkswagen models.
Having had this pristine W211 E-Class for about eight years, despite having new batteries, the battery drains if the car is idle for a few days, necessitating a jump start.
Can somebody elucidate the potential causes of this issue?
Thank you in advance for your response.

Regarding the entrance, while I am not acquainted with the saloon, if a roof antenna is there, it is advisable to inspect the cardboard liner underneath for any signs of moisture.

Thankyou, But There Is No Roof Antenna, !

Examine the areas around the rear light seals and the boot trim, then gently dab with blue paper or shop towels. This will prominently display a single droplet of moisture as dark blue, facilitating simple observation after each application. A solitary drop of moisture located higher than the water accumulation may be the origin of the leak.

It is evident that you will be applying a dab to the elevated sections containing seals and joints, regardless of your assessment of their quality, to conduct this brief inspection. There is little use in using the blue tissue to absorb places that are saturated, since it is evident that water is accumulating in such locations.

Does water spill from any corner of the boot lid when opening it? Potential access points I have indicated the rear lights and third brake light. The dotted line should be adequately sealed with Puraflex or silicone. Alternatively, if you are inclined, you may remove the number plate surround, replace all popper clips and sealing rubber rings, which number around 20, to the best of my recollection. Any individual is capable of disclosing information. They get mouldy and harden. A little drain is located in the centre of the boot lid rubber, under the number plate.

The primary seals and scuttles inside the boot need cleaning and reseating using Puraflex.

The battery may have a parasitic drain and should be inspected by an auto electrician.

Do not fret; just confront the situation and replace all sealing components.

Approximately £200 from Mercedes

Boot seal, boot water guides, rubber rivets, light seals, and similar components.

It is essential to address the metal lip and convert any rust, as it may seep behind the ageing seal.

I can demonstrate if you are near London and visit briefly.

I had similar issue with my W211 sedan as well.
I removed the rear lights and applied black RTV sealer to the deteriorating foam seals prior to their reinstallation.
I further modified the drainage canals.

What is the significance of 'the ancient giant seal'?
Are you referring to the whole boot seal? When you remove this from the metal lip it rests on, it is typical to discover that the lip is rather rusty and requires treatment beforehand.

Indeed, the rubber contracts and fails to create a secure seal. Frequently, leaks occur under the seal bead at the lower section where it curves; this happens when it is tugged by an external force or when it is extracted while pursuing a leak, resulting in an exacerbated issue.

The new seal will include a novel jointing compound, and the metal lip must be clean for good mating.

Previous discussions on this topic may be helpful.