I have had drips onto my 4/4 trans tunnel too. I have taped the grommet that cables use to go through the firewall and am waiting to see if that was the source. gaffer tape test before final solution. I don't think its the washer jets - the grommet is in a likely spot though as its position seems to be about right.I could look uder the dash while a hose is used as "rain" though.
Try silicone around the grommet holes?
G
