The only "good" way to check the bulbs is with an ohm meter or a continuity tester.
There are a couple larger bulbs in there for general illumination of the cluster, unsure if these are also inop or not.
At first I was thinking the dimmer is turned all the way down, but that doesn't explain the exterior temp still working OK.
There are 3 bulbs behind the MFI (multi function indicator), one left, one right, one center, but in TWO rows. One row is clear, one row is red bulbs, this is for when the MFI has a RED warning display. It's rare to see a bad red bulb, concentrate on checking the clear ones, which I believe is the bottom row of bulbs behind the MFI.
The general illumination bulbs I believe have yellow sockets, the sockets are removed and the bulbs replaced seperately. I belive there are only 2 of these type of bulbs, I may be wrong, if I am, then it would be 4 of these type, either 2 or 4. DON'T use regular 198 bulbs in those sockets, you could damage the cluster due to higher amperage draw, plan on getting all required bulbs from an MB part supplier, like your local dealer or Fastlane (this site).

Gilly