the sunroof has not been opened in 10 years in my 240d. i am afraid that when i do open it, it will start leaking after i close it again. I am worried the rubber seals might be brittle. Does anyone have experience in replacing the rubber seal or any other tips to keep the water out?

If your sunroof isn't already leaking, opening it shouldn't change things.

The critical issue on sunroofs are whether or not the pan drains are clear. The 'seals' are actually more of a set of baffles that keep the rain from running into the sunroof pan faster than the four drains in the pan can carry the water away.

While leaves and other debris can plug the drains, the usual problem is that eventually enough dirt and dust manages to make it's way around the sunroof to and down the drain to collect and create a mud plug in one or the other of the back drains.

The two front drains can be seen if you look into in the front corners of the pan when the sunroof is opened. They drain to a point behind the upper hinge on the front doors.

The two rear drains exit in the c-pillar vent. You can usually see the drain tube if you look in the ventilation slots behind the chrome trim on the c-pillar.

To clean the drains you can use a piece of heavy plastic cord from a string trimmer or a brake cable from a bicycle in a 'roto-rooter' fashon followed up by flushing the drain by carefully pouring water into the pan on the side you are cleaning. Depending on if you are cleaning the back or front drains, it's a little easier if the car is angled slightly down (front drains) or up (back drains).

Using the 'roto' method, push the string into the front drains from the pan side and work them down until the string pokes out from the hinge. If you feel any resistance, twirl and poke the cord until you push through the obstruction. Follow with a flush of water until it flows clear.

The rear drains have to be cleaned from the exit end. Use the same string trimmer cord and push it up the tubing that you found behind the c-pillar trim. If you feel any resistance, twirl and poke the cord until you push through the obstruction. Follow with a flush of water until it flows clear. If you start feeling like you are a proctologist or a large animal vet, you are almost finished.

When doing my rear drains on my sedan and wagon, I've never been able to get the cord to appear in the pan so I have to guesstimate when I've pushed the cord far enough.

In rare situations where there have been unexplained leaks, removing the headliner or front reveal molding has uncovered rusted-out sunroof pans or rusted/perforated tubing snouts.

Here are some associated links:
Weapons of Choice? Drain Cleaning!

Sunroof leak, Not again, please. "like a bad penny"

Leaking may not be due to windshield gasket

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Never a dull moment at Berry Hill Farm.