It is a silver box under the driver's side “knee” panel under the dash. I have se same "conversion" using pencil plugs and the old loop relay, but lately it won't glow until I start craning the engine over. I think my relay is to blame. Is the newer relay "plug and play" or should I just replace it with another of the old style relays?

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine