therubberdraintubesinthecowelarepluggedyourheaterwouldleakanti-freezeifthecorewasleakingyourewelcome

Had same problem on a surper charged gtp grand prix, which i still own,  all
the exact same symptoms, but it was only doing that when it had rained,
water was running down from outside where the wipers are,  and somehow
leaked inside the car, it ran down behind carpet and filled up the
passangers side under the floormat,  no wetness was anywhere above under the
matt, but soaked under the matt,  if your problem usually happends after
rains, this is it, a seal leak, bad, old , cracked, just replace all seals
if you can get to them,,,,,,,,,if this isn't the problem,  the only other
thing that it could be, is the leaking heater core, yes, you will smell
antifreeze, if you dont, then its the problem listed above,  there is no
other water, "anywhere" , that can get inside car other than heater core or
rain leaking in,  let me know if you get it fixed, good luck.l
Vetteman1999