Water leak into back seat?

Probably not a solution in your case, but I had a similar in a 740 GLE boot. Have a look at the rear light cluster seals. Mine was the actual Perspex lens itself where the two colours (red & Amber) join with a black band. Regards. Mike.

Has this been an ongoing problem or did it just suddenly occur. Assuming the trunk lid was not left opened slightly during a heavy rain, then you have to track down a small leak.

First choice would be the seals around the trunk lid - are they in place? Next I would look at the taillight assembly. Any damaged or cracked lenses? Is the assembly tight to the body?

Check the lenses themselves for many tiny cracks close together. The lenses themselves can fill up with water and it is possible to see water inside sloshing around after a good rain. This cracking occurs from the heat of the brake lights and is common.

Does the car have a sun roof? On the 240 series and I assume on the 740 series, there are drain hoses into the trunk. There are clear plastic tubes coming from the rear roof pilllars that join each other into a common drain. Check to make sure all are connected together.

BTW, it is easy to dislodge the pipes by placing storage items very close to the seat back of the rear seat.

I don't have a sun roof although I could do with one!

The problem first occured last March and the garage replaced a seal but I don't know where it was, obviously in the boot area but I have no idea where exactly. I had no problems till last week and it was raining lots so either its a different seal or something has come off/ gotton lose at some point recently.

I would appreciate it if someone could actually tell me where the seals are? as I do not actually know what I am looking for.

Also shouldn't there by a hole where water can drain out if it gets into the spare tyre well? That is what I was told but I can't see any spaces like that when I look around the tyre. thanks.

The seals are where the trunk lid presses against the body. They are usually in the form of a long strip of dark grey neoprene like material that is usually fitted to the body.

I can't see any gaps or damage to the seal around the boot/trunk lid.

I have no idea where the seals are around the lights - I need more help tracking those down!

But if its neither of those where could else could water be coming in? and what about the drainage "hole" that I heard was meant to be in the rear spare tyre space?

The seals go between the light assembly and the body. If the light assembly is broken or has tiny cracks as mentioned previously or is not tight to the body or if the seal is damaged then water can get in.

Those are two sources of a leak, but there are others. I've pointed out the most likely ones. Not all of us have the experience to be able to track down the source of water leaks when it isn't raining. You may want to consider having a dealer or an independent mechanic familiar with this car search out the cause of the problem.

I'm not aware of a hole that is intentionally left open to allow drainage from the trunk.