How do you manually pull codes from a 1989 jeep cherokee?

My 1989 cherokee has been NOTHING but problems and frankly I am sorry I bought the thing. Nothing works right on it, I get 5 mpg, I have already spent over $2,000 in repairs (stupid on my part) Recently all the fuel injectors were replaces and it is running even worse. It was suggested to me that I take it to any auto parts place and have them do a code scan on it. Unfortunately I went to everyone in town. Big name ones such as autozone,napa,shucks they all said there is no such code scanner for the vehicle and that I would have to pull the codes manually. Being not very happy I left with out finding out how to do that. Any ideas how? I hope it doesn't cost anything. I am not sure how much more I want to spend on this. I want to get rid of it but according to NADA with the condition it is in I am looking at $250.00 trade in. $250.00 is not worth my time.