You could try contacting FGW for some information, as they undertook a project recently to retrofit air conditioning units into DMU cabs. They may be willing to provide you with the name of their supplier. Pullman Group, based at Cardiff Canton, service the saloon air-con on the 158 fleet, and may be able to assist. Alternatively, some companies which supply air-con gear for vehicles include Sutrak and ThermoKing, try getting in touch with them.