check the footwells, they are the most common leak points. just feel around on the carpets for damp and trace it from there.
if you have matts in your car, they could be dripping wet too. take them indoors overnight and try and dry them out.
I leave a dehumidifier container thing in my car as I havnt been using it over the winter. it doesn't really help much tbh. when you get sun then freezing cold it will create the condensation on the windows and then it will freeze, it can happen to most cars a few times a year if the conditions are right.