I have a leak which wets the carpet in the passenger footwell

Warm weather climate where you reside ? Makes a difference if a/c is being used or not.

(1) >if the floor is wet with coolant/antifreeze,windows fogging up,sweet smell of/antifreeze inside vehicle,then more than likely your heater core is bad and leaking on the inside of the vehicle and will need to be replaced. (2) if it is hot or warm where you are located and you've been running the A/C,then it's possible the A/C condensation drain is plugged up and draining on the inside instead of leaking onto the ground underneath the vehicle. (3) your blower motor resistor is bad is why you only have high speed,when the resistor goes bad,the blower motor defaults to hi speed only..