does it look like its coming in at the front corner over the rubber seal? I used to get this sometimes after washing, i assumed it was cause our wash bay is uneven and the body didn't sit perfect so there was a small gap. Being quite old cars i imagine theres a bit of flex...

If the seal isn't getting wet on top where the sill is then its got to be coming in from higher up. Have you driven in the rain recently?

Common problem appears to be a seem where your heel rests. Water gets in from underneath when it rusts the seem and starts collecting in the soundproofing under the carpet making it rust worse. I've done small repairs there on both my Mk4's now, cabriolet and hatchback. Pull the carpet up and check straight under the peddles from the speaker to the centre console for bubbling etc.