The cable that goes from the lever to the heater box is most likely rusted on place.
You will need to tear apart the dash take the heater control out and remove the cable. Soak it with some penetrating oil and get it freed up and moving. Then put it all back together.
Its not a hard job, just a little time consuming. Take pics with your phone every step of the way, that way you can refer to them when you are putting it back together.
Nect thing you know, you will be an ACE Jeep mechanic. It just takes a little patience and the ability to ask questions on here if you get stuck with a problem.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...