I'll second the advice on just buying the rebuild kit. It's basically the moving piston part. I replaced the guts of mine on my '91 420SEL (W126) a year ago, more or less, at about 170k miles. Symptoms were basically intermittent periods of no heat.

The monovalve is basically a pulse-width modulation system, that is basically either fully open or fully shut. Full open corresponds to full heat, full shut corresponds to no heat at all. For anything in between, the valve opens and closes at the appropriate duty cycle. You can do a fair amount of diagnosing by placing your finger on the top of the valve body - you can feel the valve "cycling" if it is working properly. In my case it would sometimes open up if I set my ACC "temperature wheel" to full hot, but it wouldn't cycle at normal termperature settings. What tends to fail on these is that coolant seeps past worn seals, leading to corrosion. No power corresponds to full-open (i.e. full heat) = a fuse failure of the ACC system results in full heat. If you put the ACC wheel in the opposite state, you should definitely feel the valve move when you remove and re-apply the electrical connection.
I think it was this forum where I searched and found a step-by-step set of instructions to installing the rebuild kit, but it was fairly intuitive.

A couple of tips, from memory:

- I recall moving one or more things out of the way, to get more access or working room above the valve. But now I can't remember what I moved. Maybe it was a switch that detects the hood being opened? I might also have moved the windshield washer fluid reservoir out of the way, and maybe even taken the battery out to get more working room. But I can't remember for sure.

- You want to remove the coolant reservoir cap to release pressure from the cooling system before taking the top off the valve. BUT (and this wasn't mentioned in the instructions I found), put the coolant reservoir cap back ON, after releasing the pressure but prior to actually opening up the top of the valve. Otherwise a lot of coolant will drain out onto the floor. With the cap put back on, very little coolant will leak out.

- I forget the details now, but be very careful with removing the electrical connector, so as not to crack the plastic of the retaining clip.