heater core bypass

Look from the engine side and you'll see two hoses going into the firewall, in the middle of the engine compartment, a little bit towards the drivers side. Those are the hoses going to the heater core. I would try bypassing it from the heater hoses, not inside.

just buy one off the internet and follow this guys pictures http://www.volvo-forums.com/t22870-1998-volvo-v70-heater-core-replacement-w-pics.htm