It should probably be a mortar bed of grey mortar covering wire lathe, which would be covered by the mortar and not seen, in this age of house. Sometimes you will see the wire lathe where there are any cutouts for recessed soap dishes or such should you remove those.

Please post a picture :)

Of course water has gotten behind the tile. Neither tile nor grout are waterproof. The waterproofing is done before the tile goes up.

My first thought was, how did you fit a shower inside your car?