as gadge said, could also be the heater on off valve or the steel tubes that pass through the channel that may be weeping, if its coolant you will notice you have to keep topping up, if your coolant is ok it may be coming from elsewhere, check around and make sure that little (looks like a fuel can plastic nozzle) pipe that goes from the intake to the wheelarch is not blocked, rust around the window aperture is common on these little cars I had to replace the scuttle panel on mine and rebuild the a posts the rot had gone right through to the dash, check the jacking points for corrosion and the wheelarches for any holes, just look everywhere but start with the hoses and heater, for the places you cant see wipe kitchen roll round them, wet sheet indicates leak, as for the rear carpets, take them out and look for any corossion it may start at the rear wheelarches and travel through check the seat mountings and jacking points, these are great dirt traps and rust for fun, simple fixes though if you have the right tools, my gut feeling is the hozes, 30ish yr old rubber and old heater matrix, good luck