Installing rear heat/ac '95 G20 conversion

I recently bought a 95 G20 conversion (48k miles!!!). It has a control for a "Rear Fan" but no rear heat or a/c. Does anyone know how much it would cost or what is involved in installing heat/ac for the rear? It has a blower inside the rear passenger side behind the back bench and it has vents in the ceiling for middle and rear seat. I'm planning on driving from Illinois to Florida next summer and don't think the kids could stand 18 hrs without a/c. Any help is appreciated.