Amplifier broken & keyless entry intermittently broken: water damage?

[Estoril]

Hi,

I've been running into intermittent issues with my 2002 E53 4.6is where there seems to be some kind of short circuit in some electrical components. The keyless entry issue has been going on for a while and managable, but since yesterday my speakers suddenly stopped working. At first the sound became terrible (sounded far away) and started making some noises. Then it just died completely and now it makes no sound.

A few weeks ago the floormats were suddenly completely wet. I tried the mats out best I could. It happened after it had snowed all night and the next day the little bit of snow melted. Another car of mine also had a lot of water leak inside via the door rubbers that same day.

I figured I'd keep an eye out if the water issue would return, but it never came back. And now we're at a point a few weeks later, everything is still dry. It has rained a LOT lately, still the mats are completely dry.

I'm starting to suspect the sunroof drain being clogged. So now my question is where do I start looking or taking the car apart to rule out if this is indeed the issue?
For example, is there a usual spot in the car where water piles up if there's a sunroof clog?