Not sure whether you have a 940 or 960 series, but on the 240's the heater control valve is up underneath the drivers foot well kickpanel, up against the firewall by the heater core. If you have a 940, it is probably there. If you have a 960, let me know where you found it. Ordinarily, I would not fix this unless it is leaking, which you would know about... you wuld have coolant at your feet or in your carpet nearby. I don't think the latter models fail like the early 240's. It isn't to hard to replace, given they are over the footwell, but I know that Volvo engineers, in their infinite wisdoml had them assembled (probably before it was dropped into the car) with the mounting screws facing the firewall. One of those 90 degree screwdrivers comes in handy.