Seizures can be stimulated by flashing lights but usually the flash rate has to be fairly high. That being said, someone with a relatively low threshold can be stimulated into seizing by the relatively slow flash rate of most emergency lights.
As to module / engine batteries, ambulances usually have two separate power systems. One set for the engine and running lighs (basically all the lights and electrical stuff in a typical car) and the "house" set of batteries for powering all the stuff in the module. These power systems are supposed to be separated except under two specific situations: when the motor is running and alternator power is available to charge the "house" batteries or when the "engine" batteries are dead or nearly so and you must connect the "house" batteries to the "crank" batteries to get the engine to start up. There are high current relays and diodes and such that make these things work. The takeaway of this is that normally you don't have to think about charging the "house" batteries as normal ops will keep them charged.
Unfortunately batteries do get old and they don't want to take a charge or you'll have something in the module that slowly drains power away or sometimes the module batteries just self-discharge if they're not charged often enough. Sometimes the relays and diodes fail and allow power from the module to also power the "truck" side of things and that can drain both crank and house batteries very dead quickly.
The "module" disconnect cuts power to the module by disconnecting the house batteries from powering anything. That basically isolates those batteries entirely but you also lose all the power in the module. That can stop any parasitic power drains from happening while the engine is off. Often thats the loud CLICK you hear when (or shortly after) you shut off the engine. It's also often a loud CLICK noise that happens when you turn the ignition "on" but before you have actually cranked the motor. Some ambulances automatically do this, others require that you manually turn the module "on."