the ac blow out only from the frost vents

If the air is only coming out of the defrost vents, there is an issue with the ventilation system where it directs flow to the different vent positions: windshield, dash and floor. This operation is vacuum controlled. Therefore, with troubleshooting, you may find a disconnected or displaced vacuum hose under the dash or under the hood. The mechanism / motor that uses vacuum to operate could also be in question.