I've had 8 months going around in circles trying resolve a leak in the passenger footwell (left hand side - UK). Tried all sorts, thinking it was air-con, windshield, drains, thinking it's solved and then more water. Absolutely sopping wet at times and at worse also a film of water over speaker grill and carpet under door soaked. I fixed broken cables yesterday that solved "ignition lock faulty", footwell was dry then, soaked today, only difference was it rained heavily in between which made me look at the sunroof drainage. Pushed a speaker cable down it, got stuck after about a metre. Poured water in the sunroof channel, a few drips came out at most.
I know the the air compressor method is frowned upon for being too harsh and potentially blowing a joint but came up with this tool to do it more gently....and didn't care by that point, I would have tried anything! Got my son's ballon pump, wrapped some tape around the end so a caulk tube's nozzle could screw on. Then a few wraps of amalgamating tape around nozzle tip to make a rubber seal to make it air-tight with sun roof drain. Pumped away a number of times, heard a squelch of something shifting and it got a lot easier. Poured a jug of water in sunroof channel and it poured free out the bottom of wheel arch drain.
I'll keep my fingers crossed that's the end of a smelly wet car.