Pull up your driver's side floor mat.

If it's wet there or there is a buddle of greenish yellow fluid underneath, it's likely the heater core. (Or the "O" rings.)

-BTC

'98 V70 T5 5-speed, 167k mi, front IPD stabilizer bar, rear factory HD bar, Bilstein HD, Volvo strut tower brace and skidplate, e-codes, XC grill, Lidatek LE30, V-1, Mobil-1 since new

"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." -- Douglas Noel Adams (1952-2001)