Assuming the 603.971 glow system is similar to the 603.961 glow system:

Glow system self-diagnostics are pretty good in these cars. If the glow system isn't up to snuff the glow light will tell you by not turning on at all, staying on af the car has started or blinking.

If the glow light doesn't come on, check the fuse strip in the glow relay. Check it well because sometimes those things crack over time but still look intact.

If the glow light comes on, check the resistance of each glow plug to ground. You can do this at the glow plug relay. The connector with 6 pins has the wires to each of the glow plugs. I don't know what the acceptable range is for a 603.971 but they should be within 0.1-ohm of eachother. I mean it's unlikely that they're all bad. A bad reading could also be a bad wire. If you find one out of spec, check resistance at the + terminal of the glow plug.

It could be that carbon is fouling the tips of the glow plugs. You'll have to pull the plugs, clean the tips and use a special reamer to clean the glow plug channel in the prechambers. Ugh.

Sixto
91 300SE
87 300SDL
83 300SD