It is probably time to clean the water drains in your Cayenne 958. It is very important to have working drains, or you will risk costly water leaks into the cabin.
Engine bay air box
Any water between the windscreen and bonnet is drained through a rubber hose on each side of the main ventilation air box. These hoses are placed behind the fibre wheel arches. These drains are common to clog up and prevent draining of water. It is possible to store about 10 litres of water before it starts to splash into the cabin.
It is therefore important to keep the drains open and free of debris, a job that is done in 20-30 minutes.
The drains are unfortunately mounted in a way that debris will easily block the opening, especially on the left side with the external heater unit piping, if fitted.
Start by locking the air suspension, lift the car and remove the front wheel. Dismantle the Torx screws that holds the rearmost part fibre wheel arch lining. The lining can then be bended forward to provide access to the drains.
The drain pipe outlets are made of soft rubber, and it is easy to pull them out of their seating. Please be aware that a LOT of water can pour out of the opening once the drain is removed. We had 5-6 litres of water coming out on both sides. There's a similar drain outlet in the right side wheel arch. Check that the drain openings are OK by pouring some water through the latticed openings below the windscreen.
Most Cayennes have the glass panorama roof. Any water that sips through the seal shall be led to a gutter with outlets in front and rear. The roof hoses can be clogged by dirt and debris, especially the rubber outlets at the rear.
It is especially important that these drains are not clogged, any water overflow from the roof will pour straight down into the electronics placed behind the right carpet wall in the luggage compartment.
The two front drain hoses end up in the engine compartment main ventilation air box, so you have to do th eabove mentioned work on the rubber outlets before doing any drainage tests on the sunroof.
Park the car at an angle so any water is lead to the front drain holes. Use VERY LITTLE water to test the drains.
This is important to avoid any overflow into the car interior.
This is the achilles heal in the sunroof drains, and leaks are most often catastrophic. If water overflows in the sunroof gutters, it will flush down into the audio amplifier and Rear Control Unit monted behind the right luggage compartment wall. Any water/corrosion damage in these units is most often not repairable and very expensive to exchange.
To test the rear drains, park the car in a slight angle to direct any water in the sunroof gutters to flow backward to the rear drains. Also here it is very important to use VERY LITTLE to test the drains. This water shall be drained out just behind the rear wheels. You'll see the rubber studs if you loosen the rear of the fibre wheel arch lining.
My driains were fine, but I used a small garden can, with the result that almost all the water spilled down onto my electronic units. This was the opposite of what I wanted to acheive!
Luckily, I was able to save the amp and control unit by very quickly removing the carpet wall, disconnect and remove the units as well as drying them thoroughly on my antistatic electronics workbench.