The obvious causes for damp/wet inside the car are the door seals and, while these can get damaged, it's rarely the source of the problem.
The boot seal can sometimes get damaged but it has to be in a pretty poor state before water finds it's way inside, other when you're directing a pressure hose in the area
The three most common causes of wet in the car are blocked drains
The gutter surrounding the boot lid drains down behind the rear light clusters. Pour water around the bootlid and you should see it appearing below the centre of each rear light. This route can just blocked by debris (or an over-enthusiastic attempt to keep water out of the lights) such that it overflows behind the lights on the inside of the boot. You'll need to remove the light units and clear this path if yours is doing this.
The front door window seals are only intended to guide the glass and stop snow getting inside the door, they don't stop water. Any water from the door glass should drain inside the door and out through drains underneath, which you can see with the door open.
If these drains get blocked by debris (or waterproofing wax, etc) then the door will start to fill with water until it reaches an opening behind the door card, at which point it runs down the back of the card and into the carpet.
One members car that we worked on together used to steam up the windscreen as soon as the engine was hot, in anything but the driest weather.
We tracked this down to his drivers door drains, causing the carpet under the seat to become saturated, which would then be heated by the proximity to the cat.
When we poked the drains about 3 litres of filthy water ran out onto the road
The sunroof seals aren't weatherproof, none are, and manufacturers know this so they include a gutter in the design with drain pipes that run down the A and C pillars and out through the wheel arches.
These pipes are quite thin and can get blocked with muck.
When the water can no longer drain away it overflows the gutter and runs between the roof and the lining. It can then end up anywhere in the car, depending on the slope.
If you have a sunroof and the car is generally damp inside, then this is a likely culprit.
Other sources of potential leaks include the front and rear windscreen seals, especially if the glass has been replaced at any time.
Damp can also accumulate due to lack of ventilation, the most common cause of which is blocked cabin air filters.
Running the a/c in damp weather can help keep the glass mist free, as can making sure that the inside of the glass is really clean.
If you're having to use demisting products on the glass then there's something else wrong.
The a/c can also be very effective in drying the inside of the car, using recirculate if it's really damp.
In warm damp weather you should see water draining from under the car when parked, after the a/c has been running.
If you don't, then the condenser drain might be blocked and this can cause issues of its own.
One member reported wet inside the cabin from a pin-hole leak in the heater matrix.
This was a rare occurrence and was accompanied by "a hot engine smell".
Beyond this list the sources of damp in the car start to become more specific to your circumstances, e.g. a nervous puppy
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