The glow plug systems on early W123 cars, mostly 1980 and earlier, have a glow plug relay that is underneath the dash within the cabin of the car. In these systems the fuse for the glow plug is located within a separate fuse box attached to the firewall of the car. The fuse can break if there is a short within one of the glow plugs; the fuse will need to be replaced after the glow plug is replaced. In other cases the fuse box itself can be damaged by excess heat or other conditions and so the fuse and the box will both need to be replaced. Note that on newer W123's the fuse is located inside the glow plug relay, which is located under the hood on the driver's side fender. This guide does not cover these cars.