Air conditioner not blowing cold air

Boy, that is a lot to go on. Have you checked for leaks and low refrigerant?

Not yet, we are just trying to get some advice on the most common steps to take in order to try and narrow down the most likely cause of the problem.

Given the age of the vehicle, loss of refrigerant can be expected. Your mechanic can put some in the system, then check it for leaks. The problem with GM and Ford systems is there is no way to know how much refrigerant is already in the system except to draw it all out, then pump in the measured amount called for. Without doing that, you run the risk of over-charging the system which can be more harmful than leaving it under-charged.