Leaking cores are fairly common on these cars. I'm on my third (one warranty replacement at 3 years old, one replaced by me at 6 years old). At that rate, I should need a new one again in about two more years.... FCP Groton has reasonably priced replacements.

Always replace the O-rings when you're replacing the core. They're cheap, and they'll save you from a common source of leaks.

I would think that if it's leaking again, you'd be able to see it on the console hump if you pull the trim out and pull the carpet back.

--
Jim Rothe, '99 S70 T5M,
http://www.jimrothe.com/volvo/index.html