Final answer:
An alcohol injector should be installed between the compressor and the air dryer in an air brake system to prevent freezing by mixing with moisture before the air dryer removes it.
Step-by-step explanation:
When installing an alcohol injector on an air brake system, it is important to locate it correctly to ensure the system operates properly. The alcohol injector should be installed between the compressor and the air dryer. This placement allows the alcohol to be introduced into the system where it can help prevent freezing by mixing with the moisture in the compressed air before it reaches the air dryer, which then removes the moisture and alcohol mixture. Placing the injector after the air dryer would be ineffective as most of the moisture would have been removed, and installing it before the compressor or after the supply reservoir would not address the issue of moisture within the air brake system effectively.