A method of constructing modular air conditioning systems for vehicles includes providing identical universal base units that can be built out into a variety of different types of air conditioning systems. Each base unit includes a frame, a condenser, an evaporator coil, and a blower, and has open-circuit refrigerant connection lines so as to be non-operational without a completion kit. Completion kits are provided in a variety of configurations, including at least two of a simple connection kit, a front box kit, and a compressor kit, each of which is connectable with a universal base unit to form a unique type of air conditioning system.