Why is the carpet wet under the driver and front passenger's feet?

the door seal is letting in water i think

If it smells like coolant, the heater core could be leaking. If there is no smell, it could be coming from a bad seal in the cowl area or rust holes in the firewall.

I have this problem when I park on my folk's driveway which is inclined up, or slopes up towards the garage. It has only occurred in the summer. My dad said something about there being an AC drain for the condensed water collected (by the AC being turned on) that might be plugged, and when on an incline with the nose angled up, it might be draining onto the front floor. Never had any water on the driver's side, though, so your problem might be a leaky heater core like 13255 indicates.

thanks everyone, I've had it diagnosed & repaired($700). It was rainwater coming through the rusted firewall. Fenders came off & the dashboard came out. Sealant applied inside & out. Seems to have done the job