heres a tricked i used in my grandam. i used those dust removers for your keyboard, if u turn it upside down u get a white spray instead of just clear air, u can roll ur windows up and spray the inside shooting up and where ever it comes through thats the prob. if not that can be crossed of the list. im sure it doesnt work perfect, but it helped me :)
BEWARE: When using the can upside down, it freezes whatever it touches, therefor spray it seconds without trying to freeze the weatherstripping. try it on ur jeans to know how long it takes to freeze
do that or just get and air gun and use the air to shoot through it and put ur hand on the other side to feel if any gets through.