My driveway is sloped and for the 14 years I owned it, from new , my Yaris was prone to excessive condensation. I never did find the source , which doesnt help you much
. It never extended to wet carpets as such.
I'm fairly sure it was related to the fact rain water sometimes pooled on ledges etc which would have drained themselves had the car been parked on the level.
Water can sometimes trickle quite a long distance from where it leaks in.
You could try getting the car dried out as thoroughly as possible. ( Easier in a hot summer
) and search various nooks and crannies to check they are currently dry. (and crooks and nannies if you want) Then put a hose on the car in strategic areas and check again for dampness. The reason for the before and after is it might help you trace the progress of new dampness/wetness as its spreads from the leak. I find dry fingers are a good judge of dampness, perhaps aided with a bit of something like talcum powder.
I dont know if there is any damp meter that could aid in this. I have experimented in the past using an ordinary digital multimeter set on 2000k ohms. When you place the probes near each other ,but not quite touching , there is sometimes a slight trickle of electricity and a reading if if its wet (battery voltage) But never found it any advantage over dry fingers (I think damp meters with probes work on the same principle, but presumably they are much more sensitive.)
