The heater core is buried in the dash, but the control valve is inside the console near the gas pedal. Drain your coolant, then remove the driver's side console panel. Follow the cable until you find the valve (you will see the heater hoses and a copper wire running into the valve). Relatively easy to replace.

No offense, but are you sure that you are operating the heater controls the way those goofy Swedes intended? In your last post, it sounded like you had the heater control on at the same time as the A/C. You want to make sure that the heater control level is pushed all the way to the left before turning on the A/C. It could be that the cable needs to be adjusted such that even with the level maxed out to the left the control valve is still open.