Agree with the hatch seal diagnosis, but offer the following:

If you have wet front mats as well, then you probably have clogged drains on the outside of the blower cavity, and/or rust-out inside the blower motor cavity. Rust occurs there because the plastic cover warps and allows water draining off the hood and windshield to get into the blower cavity, where there are NO drains. So it rusts and then there is nothing to stop the water. From there, it's all downhill to the rear footwell.

Suggest cleaning out the drains and replacing the blower cover and gasket. You may also get to spend some quality time fiberglassing the rust holes in the cowl area as well.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe