Took my carpet out to replace it and found a stain on the driverside floor...does anyone know what could cause this? Previous owner was a pig and the carpet was stained all over. Im not leaking oil or anything. Maybe heater core issue in the past? I tried to scrub it out but no deal...just want to check everything before i put the new carpet and seats in.

 

First thing coming to mind is clutch hydraulic fluid. Maybe fluid leaked from clutch master cylinder diaphragm at some point? Pretty common problem

Looking back at the pics, I'd guess related to heater based on how big the area is.