Avbuff,
This is an ongoing issue with the 135 that is yet to be recognised / accepted / adressed fully by ECD.
There is a GSE cover for this vent available now to stop the weather from going in one side and out the other when parked and for aircraft washing.
This is a good source of info.
There was a recent Customer Conference in Europe and a briefing of that is available electronically. Get your friendly Tech Rep to supply you with one.