I'm pretty certain this test convicts the pump. If you have DVM, check the ohms on the pump. I forget the correct value, but if you get a small number, perhaps 1-2 ohms, that's your problem for sure.
I've always thought that pump was pretty darn steep for a small electric motor and some plastic. Never seen it for less than $99. Since it does not get too cold down here in Dallas I just let it go, never really missed it.
I can't comment on the 126 chassis, so far I've only suffered through ownership of 123, 124, and 210 chassis cars
If I was a betting man, I'd say a shorted aux water pump damages the PBU on a 126. Both the 126 and 123 were designed in about the same timeframe, so I'd expect similar behavior. When they did the 124 ~10 years later the designers had learned the PBU needed protection from the water pump.
Where did they hide the pump on the 126? On 123 & 124 cars it's pretty much open the hood and unplug it. One minute of work, one minute to wash your hands...
Feel free to mail down some Pacific Northwest micro-brew!