Unlike the AC evaporator, the heater core is actually almost easy to get to. However, since the cabin air intake is pretty close to the back of the engine and the turbo oil return line, I'd make sure it's the heater core before taking the center of the dash apart. Another clue (I've had leaky cores twice) is that the antifreeze accumulates into a puddle under the driver's side floormat.

I think Volvospeed has some detailed instructions and pix of the heater core job somewhere.

-BTC

'98 V70 T5 5-speed, 171k mi, Bilstein HD, front IPD stabilizer bar, Volvo strut tower brace and skidplate, XC grill, C70 jewel E-Codes, V-1, Lidatek, Mobil-1 since new.

"Human beings, who are almost unique in having the ability to learn from the experience of others, are also remarkable for their apparent disinclination to do so."-Douglas Noel Adams (1952-2001)

yep, the heater core. Not too bad of a job.

Drop both underpanels on drivers and passenger side. Slide the center counsel back (must remove 2 screws holding it to floorboard in the back of the counsel). Then remove the 2 screws that hold the housing around the heater core, pull apart and the heater core just pulls out. Make sure you have lots of towels on both sides as some coolant will come out. Bay13 may have some detailed instructions. I just did mine and got the core from eeuroparts.