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This is EXTREMELY difficult, because the gauge wasn't meant to be disassembled. Nonetheless, it is possible...but not for the faint of heart. Read on at entirely your own risk. WORK ON A SOFT PADDED SURFACE. A 3x or 5X jeweler's loupe is very helpful if you are over 25 years old. Go to Harbor Freight and get a cheapo set for $3.00. It will be among the best $3.00 you've spent.
The front bezel of the gauge is apparently crimped (permanently) on the gauge cup, which has a ridge around the front. It can, with a great deal of patience and effort, be lifted around the edge VERY SLOWLY with a small long flat, thin-bladed screwdriver with maybe a 1/8" blade, no more than 1/32" of an inch at a time. Do NOT distort the front of the bezel, and go around the circumference (several passes may be required, bend as little, and as often, as possible) until the top will just barely pop off. The goal here is to distort the metal as little as possible (NOT the angled front) so that the bezel can be removed and reinstalled with a "snap". The lip of the gauge cup is no more than 1/32" of an inch. Do not allow the screwdriver to slip!
After popping off the bezel, the glass can be removed flip over and let it fall on the carpet, towel, etc). Then, there is a light shield to be removed that may have been glued at the very, very edge. Remove the four screws on the back, then by LIGHTLY (LIGHTLY) tapping on the rear connector, about 1/2" above a carpet or bundle of cloth, the light shield and the "guts" of the gauge will come out.
From then on, painting the needle goes like the you would do a speedometer. MASK with GOOD, NEW masking tape under the needle. I didn't, and should have. The matte face of the gauge is easily damaged. I thought about changing the lightbulb but it looks like it's in good shape and appears to be a very long-lived, grain-of-wheat type bulb. It is soldered into the board, and replacement would require removal of the needle and face place (two screws and a very hairy pulling off of the needle). 12V bulbs like this are available in hobby stores. They are usually designed for long-life and may have a more reddish hue because they are "underdriven" at 12V, which contributes to their long life. Be aware that using a too-bright, and therefore, hot, bulb will make the gauge less accurate!
My gauge's bezel was rusted slightly and the paint was coming off. I sanded it with 320 wetordry and repainted with a matte-ish, wrought-iron paint. When sanding, keep in mind to minimize the areas that were bent during removal. The original paint was fairly glossy. I allowed the new paint to dry for TWO DAYS before proceeding. If you put your nose to paint and can still smell it, it isn't really dry. You will need to put a little pressure on this part to get it back together, and you don't want to ruin the paint (I suppose that you MIGHT be able to do this by masking after reassembly but I think it would be more difficult to do neatly).
While the bezel is drying, cover everything else up with a dust-free cloth.
When reassembling, cleanliness is CRUCIAL. NO dust is permissible, in the cup, in the mechanism, on the face, or on the glass. Hold everything up to a bright light before reassembly. Replace the four screws on the back first. There are guides that help position the gauge in the cup. Do not overtighten; you are screwing into plastic. Now replace the light shield and glass. No fingerprints inside the glass!
If you have removed the bezel properly, it should snap back on with your guiding it around the lip of the cup (using the loupe helps a lot here). There is a gasket inside of the bezel that seals the gauge and makes the glass rattle-free. Any marks or bends that were made from your removal behind the 45 degree part of the bezel should be very difficult or impossible to see when installed.
So, that's it: proceed at your own risk; removal of the bezel requires a touch and I'm not sure that most people could do this successfully. You may ask, "why do this"? Well, because the day will come when you can't buy a replacement, it's good to know, and I figure I spent $25 for the gauge, maybe $3.00 for the paint. I spent at most 3 hours on it, so I effectively paid myself $70 an hour, tax-free as opposed to buying a new one. Now I can buy more MB stuff that I can't restore!
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