The fuses and relay boxes are built and supplied by Freightliner but the coach manufacturer mounts them wherever they must to fit the coach's layout.
On my '04 Vectra (co-model to your Horizon) the fuses are at the very rear (driver's side) of the coach in the same bay as the side radiator. The bay door needs to be unscrewed and swung open (and propped, no struts.) The fuses were in a cutout opening on the right. There are separate fuses for the lights and left/right brake/turn. As I recall, they were 20a ATM fuses (the smaller blade type.)