water on the electronics can cause all sorts or bizarre issues, so it is possible that this was your problem all along
I am surprised that your garage could not diagnose this.
As far as advise, if it was a main dealer doing the work, I would involve BMW customer service and complain and complain and complain to whoever will listen and run the issue up the chain as far as possible. The more you complain, the more likely you are to get something from them.
You can also complain to the management chain at the dealer.
If it was a non dealer or non certified by BMW garage, then all you can do is complain to the management there.
your best bet is if it was a main dealer, to complain to BMW. They don't like it when a customer gets the runaround and poor service like this from a dealer.
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