I had a TDi 200 disco. Behind the n/s A panel .............. where the passenger left foot might kick............ there were plenty relays and so on. If a 300 is similar that's a good starting point.
I think the alarm is standard on a 300 and can cause problems even when it's bone dry.
If you tested the snorkel thoroughly and had water coming over the bonnet then water has gone down the heater intake. This happened to my friends 300, I er may have been driving
To dry the carpet and so on was a big job as we had to remove the seats and ended up cutting the trim underfelt at the transmission tunnel so we could remove it all as two halves and then clean it all. I think we were at Glentarkie and the water was very dirty!
Like you though we found the snorkel worked splendidly