Id def keep the AC too unless youre doing it for a race oriented car. When I bought my car it had factory AC installed... someone just decided to take it out. Big ol ugly AC bracket was still there though. Trust me I wish it was still installed. Its already hitting 90 out here in Mass and with the humidity, it seems a lot worst. I didnt mind it when I lived out in Cali though I lived right on Huntington beach more or less. New Mexico was pretty great too with 0 humidity... but anyway, yeah the defrost will still work fine. Unless you have a bad heater core like in my car.. thank God its summer..