replacing leaky heater core
 

I just got done replacing the blower transistor on my wife's '01 S40. While I was down under the dash, I noticed some coolant stains on the carpet. Now I understand why her car has seemed to have a sweet smell lately. Anyways, I'm guessing it's a leaky heater core, o-rings, or gaskets.

Has anyone here experienced a leaky heater core or o-rings? Is it a pain to replace?