1984 Volvo 760

Heater problem
1984 Volvo 760 V6 Two Wheel Drive Automatic 100, 00 miles

I hope you can help me, the problem is I have barely no heat in the car, but some. I have had the thermostst changed and there is no difference. The fan does work. I m wondering if its the climate control system?And how will I know if it is . I have had no heat in this car for 4 years and I m at my wits end . Would appreciate hearing from you, thanks. From frosty c in wpg.