drain plugs on mazda 6?

If there is one it would be under the carpet large grommet.
However odds on the carpet will be held in place by grippers on the door appature edge and tucked under rear seat.
See if you can borrow a carpet cleaner or aquavac to vacuum up water.
Water ingress may well be through the door closest to where the water is collecting. Membrane behind door card / panel may have come loose or damaged allowing water into the car.

Maurice
February 2011