Unfortunately, I cannot tell you the exact amount of refrigerant needed for both the A/C units (assuming you mean the heater and A/C). The amount varies depending on the specific model and condition of the system. You must consult a repair manual specific to your 1994 Ford Aerostar model to find the correct charge amount for R12 (which you cannot obtain) or the equivalent charge for the R134a replacement. A qualified technician will also be able to determine the correct amount using an A/C charging machine with gauges to measure the system's pressure and temperature.
Never attempt to work on your vehicle's A/C system yourself unless you are properly trained and have the necessary equipment and safety precautions in place. R134a is under pressure and requires specific handling. Improper handling can lead to injury or damage to the system. Take your vehicle to a qualified technician for A/C service.
