Most afterglow systems run the plugs at half-voltage or reduced voltage after the engine starts for some period of time determined by a timer or by a temperature sensor. The purple wire mod doesn't do this, it runs the plugs at full 12V until the timer runs out.

The plugs run MUCH hotter when they're powered vs when they're being heated by the combustion gasses. The longer they run, the more wear they take on. They do have a finite life.

If my engine needed afterglow to run halfway decent, I'd mod my system in a heartbeat. Glow plugs are cheap enough to replace that the reduced life wouldn't bother me. Luckily my car is pretty easy starting so I don't have to worry about it.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]