I've taken out many a cluster before. Taking it out isn't really all that hard. All you need to do is remove the kick panel, push the sucker from behind (leaving you enough room to get your hands behind the cluster), and start disconnecting cables. On a W123, most of the connections are kind of idiot-proof, in that they only go in one way / to one location. As a previous poster mentioned, however, make very certain that you resecure and tighten the oil pressure line BEFORE starting the car back up. The pressure line is a mechanical one, and will spray black oil all over your interior if you forget to put it back on. Also, when you get everything plugged back in, find the speedo cable down by your brake pedal, and make sure there isn't too much slack behind the dash. Too much slack can mean a kink, and a kink can mean a broken cable. Trust me, I've been there.

That being said, mine is still doing it when cold, and even though I kind of "expect" it to happen, it still makes me jump a bit when it does. It's supposed to be a whopping 45 degrees out tomorrow, so maybe I'll fix it in the parking lot of my parts supplier during lunch. I think the worst part of this whole thing, is that it utterly ruins my air guitar solo with a sour whine!

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]