If the A/C no longer blows cold then I would suspect the system has lost it's refrigerant charge. Being a 1990 it will have the old style (R12) and since the production of that was halted by the EPA, You will need to "retrofit" your system to the new R134a refrigerant. there are numerous steps involved and it takes special equipment so this fix will require a trip to a qualified shop that deals in automotive A/C systems. You don't HAVE to go to the dealer.