Do you smell coolant in the cabin with the heat on? Are the carpets near your feet (when you extend them out) on the drivers' side especially (sometimes it gets over on the passenger side too) wet? If not, then if you pull the carpets up a bit, is it wet underneath? If no to these questions then I wouldn't suspect a heater core...you probably need a new hose. The hoses get soft and don't hold a good seal after years and high temps.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.

Dennis
I have heard that S 70 are bad for heater core life. Did you ever switch away from Volvo coolant?? it's not reccommended. A good way to check is, having the heater on full blast through the front windshield defroster grille, if the windshield fog's up it is definately the core. My 91 740 went about a year ago, when my daughter told me, dad how come the windshield fog's up all the time? That was it, so god luck to you.
Rudi