Osram (European, Sylvania Bulbs)
[NO JAPANESE,NO KOREAN,NO 'MERICAN]
OF THE EXACT WATTAGE CALLED FOR ON THE LIGHT'S grey metal Backing Plate.
The Wattage is Stamped onto the Backing plate.

Clean ALL the Contacting surfaces of the Lighting System involved.

(Hint: the "Lamp Out" relay ONLY DEALS WITH EXTERNAL BULBS)
(ANOTHER HINT: CHECK THE LICENSE PLATE LAMPS!!!)

Parking Lamps light up when used in the CONUS style , BUT Not when used
in the E.U. Style.
Yeah, You MAY be looking at a BAD Light Switch.(FUSES +Contacts first)

Start Troubleshooting with the basics ALWAYS...
A DMM and an ETM Schematic for your chassis.

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'84 300SD sold
124.128