Inside thhe glow relay there is the fuse that is mentioned. It handles the heavy load of the glow system. This load is many times greater than any other load except for the starter.
The load causes the fuse to heat and expand. The expansion and contraction causes the fuse eventually to crack and fail. A simple maintenance consideration probably more important in colder climates. The car won't start well cold with the fuse and glow system down.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician