Hi.
First, are you sure you have a filled cooling system -- what's the level in teh overflow reservoir? If it's empty, you probably have enough air in the system so that will prevent the needed amount of coolant from flowing through the heater core.

Second, what does your temperature gauge say about the coolant temperature? Your '89 has the infamous Temperature Compensation Board, but if we assume it's working right, is it reading normal temperature? If low, you could have a stuck-open thermostat -- thermostat's fail (it's happened to me) -- and then you won't get enough heat.

Finally, how do you know that you don't have a clogged heater core. The previous owner may have neglected the cooling system, or used hard water in the solutions, and you can have a ruined heater core.

Just some suggestions.
Good luck.