On the 1992-94 W140 chassis the ambient temp guage doesn't use a replaceable bulb unless the cluster has been replaced by a completely digital one. The temp guage must be removed & a suitable bulb must be soldered to the board.
You can tell if you cluster is digital by the type of ODOMETER that the car has. IF you have the older analog style then the odometer uses white rolling wheels that you can always read the mileage. The digital one must be powered-up to read the mileage.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
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44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)