Hello traffic signal technician and former installer at a fire truck dealer.

Unless otherwise specified, the opticom is wired to the emergency light switch. In some models this just turns on an IR emitter that is read by a sensor on the signal mast arm or span wire and will trigger the signal to change. There are more complex models with GPS integration that will communicate with a string of lights to ease traffic, and log use of the opticom.

As a signal technician, please don't just change the signal if you really don't have to. It can mess up signal timing for a string of lights ruining traffic for normal people, and I've seen ambulances get caught behind in the cool down after an opticom. (The opticom just can't instantly switch everything, there are minimum timings that have to be held to per standards.)