What do you mean by not working? Do you mean the fan doesn't come on, or only at the slow speed? Do you mean the fan works fine but not heat comes out?
If the fan doesn't come on it may be a bad fuse, bad fan, or a bad resistor pack.
If the fan works fine but just get cold air, then most likely the heater core is clogged. You can try backflushing it but given the age of it may just want to go and replace it, and when doing it also would want to do a flush and fill of the whole radiator fluid system to get all the old coolant and debris out of the system. Also while you are in there may want to replace the flow restrictor and the T fitting that goes between the heater hoses and the coolant tank as both parts are plastic and known weak points in our cars. - Peter
1995 Caprice SS/9C1 Black w/ tan leather interior
1993 Ford Escort LX Mocha Frost Metallic w/ tan interior