You need to get hold of a bus company electrician or maybe the bodybuilder agent.
When new, any problems would be dealt with by Alexander Dennis until the bus was out of warranty by which time the operator
might have gained some knowledge of the system.
Some bus operators have work done by the vehicle dealer or if engine-related by the engine dealer (for example Volvo truck dealer if the engine was a Volvo).
Vehicle electrics are something of a black art since the days of simple two-wire systems and involve computers to signal functions - Canbus.
https://en.wikipedia.org/wiki/CAN_bus
https://www.youtube.com/watch?v=ZHaxv-cGZFI
Maybe a vehicle electrician that specialises in modern vehicle systems could help?