How to replace heater core?

Not too bad, it's under the dash next to the accelerator pedal. Disconnecting the lines and getting them back on and sealed properly is the hardest part.

http://www.volvo-forums.com/t22870-1998-volvo-v70-heater-core-replacement-w-pics.htm

A volvo dealer will charge $900.00 or more. Go to Volvo part and purchase a Behre core $123.50 with a discount coupon of 50%. You can install it yourselp. Go IPD and view video.