For what it's worth the W123 auxiliary water pump interchanges with aux water pumps found on many other Mercedes(part numbers below). The consensus with the aux hot water pump appears to be either unplug it to protect the climate control unit or replace it with an elbow or some other variation of "bah humbug on that pump". I wonder if that's really what the majority of owners are doing though. Lots of hits on threads about adding a 1A fuse to the pump so clearly not everyone is ditching it. It's definitely one of the cheaper and easier parts to replace. Another factor for many owners is that other people often drive the car and sometimes non-mechanics lose interest if a car has too many asterisks to operate it. If it was just me I'd leave it but if I can keep an always-shivering-wife 'on board' with little things like seamless heat for $30 then it's a no brainer. I think initially it definitely provides more heat to the cabin sooner during engine warm up compared to no pump. In the UK they call it a water pump for the parking heater. I guess if you're going to go parking you'll have better odds of getting past first base if your parking heater is in order.

When I was looking for another pump there were very few used options and they were all as old or older than my dead pump. After some digging I chose a pump based on dimensions, configuration, and the plug. The interchanges were worked out from there after the install verified everything. Interchange numbers get you a lot more choices and more importantly much newer pumps. From what I can tell the only difference is the plug location. The W123's plug into a pigtail coming off of the pump while the others plug-in at the body of the pump. It's all the same connector and so far the same polarity + / -. The pump I got was an exact fit. It quietly draws .250 amps and is 16 years newer.

You can cut/paste this into an eBay search window and sort the results from lowest to highest price and click to include descriptions in the search. It is set to exclude some of the lower quality brands. The first two numbers are my original pump's part numbers from Bosch and Mercedes. The rest are interchanges. I'm sure you can cull out a few more.

(0008357064, 0130002071, 0392020026, 0130002070, 0008356964, 0008358064, 0018351064, 0018351164, 0018351294, 0018351364, 0392020002, "3050-293593", 3050293593, "0133-1803329", 01331803329) -(mtc, uro, vollig, motorking)

Note that a seller might be selling multiple pumps and get part numbers mixed up so just make sure there are only two hose fittings and the 2-pin male plug-in. The odd pumps have a third slender hose fitting which does not apply. I suspect that third fitting on those pumps feeds a coil inside the washer fluid tank. Also, based on the volute end these will only operate correctly one way so just make sure that the brown wire goes to the negative terminal(clearly marked on the pump) or to the other brown wire if you're connecting to a pigtail wire.

For those thinking of replacing their pump with an elbow I can attest that a clear inlet/outlet chamber already acts like an elbow. While moving lines over to the replacement pump for a brief moment there was an open inlet on one pump and an open outlet on the other and coolant flowed surprisingly well from both.