In tackling my mysterious coolant leak, the next step was to replace the heater core. I obtained a heater core & seals from FCP Groton. Here is what I found & what I wish I had done differently:

The flat rate for a heater core replacement in a 1999 V70 is 4 hours. Warranty pays the dealer 2.4 hours. It should take a dealer no more than 90 minutes, and the rest of us about 2 hours.

When I removed the panels that give access to the heater core, there was coolant very obvious & I went ahead & replaced the heater core. I wish I had known how easy it is to inspect the situation. I might only have needed o-rings. Had I checked first, I could have pressurized the system and seen if the leak was coming from the o-rings or the seam between the heater core & the housing it sits in. (I filled my old heater core up with water & inverted it. It hasn't shown any visible sign of leakage, but it might behave differently when hot & pressurized.) It should not take more than 15 minutes to pull out the lower drivers side dash panel, pull back the carpet and remove the plastic carpet support to be able to see the heater core & it's connections. You can also remove the radio & reach in and feel the bottom of the heater core & its connections. Beware, my arm came out of that exercise with a 5 inch bloody scratch.

There is a torx screw between the two pipe connections at the bottom of the heater core. You can't see it & it is not clearly indicated in any of the manuals (Haynes, Volvo, alldatadiy.com). Someone on the brickboard mentioned breaking the plastic block during heater core replacement, and this screw is probably why. When removing the 4 obvious screws did not loose the heater core, I looked at the new one & saw the hole between the pipes, so I knew to look for & remove the screw, but I can see how someone could break the plastic block.

After I reassembled the heater core, there was a big coolant leak from the o-rings at the pipe joint to heater core connection. THE GREEN O-RINGS SUPPLIED BY FCP GROTON are too thin. Since the local Volvo dealer closed & the nearest one is now 25 miles away, I went to the local parts store with my old o-rings. They did not have metric o-rings, but they had SAE o-rings that were close enough in diameter & thick enough to seal properly. Be aware that the o-rings must be properly seated in the grooves of the holes at the entrance to the heater core. Just slipping them onto the pipes and pushing the pipes into the heater core is not enough.

I did not drain the cooling system. I squeezed the heater hoses shut with small vice grips. I also used some plastic painter's tarp material + newspaper to absorb the coolant that drains out of the the core when you disconnect it.