Well,,,,
Mercedes dealership..... about $400.00
You should diagnose weather it's the guage at fault or the temp sensor. The sensor is only $50.00 from the dealer. ( you can get the correct resistance reading for your temp sensor to diagnose if you do a search on Mercedesshop.)...
On the other hand, if the digital readout screen, LCD, is blacked out or displaying chopped digits,,,, the guage needs to be changed. If the display is showing a blank, it could be the sensor or the display has bad ground connection. Either way I wouldn't pay dealer prices to fix it. You can troubleshoot it for less than a quarter of the cost they charge.
Why do you need it fixed by tomarrow??????
It's not that accurate/important.....
Good Luck!
Adam

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Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320