OK. I have the cables... but no box. And I can't begin to imagine why someone would have removed it.
I stripped away some of the insulation to expose the wiring. One is pink with black stripe and the other a light green with a black stripe. I presume each would have connected to opposite ends of the missing box.
After switching the A/C on to high, I touched the wires together and got a very faint spark, which was only visible because it is dark outside. This would indicate a weak circuit is being completed in the blower motor circuit, but not carrying nearly enough current to turn the motor. I assume -- and hope -- this is because the brushes are worn.
Over the past year or two, the motor has become progressively weaker, and I supose I will need to have a look. There was an excellent pictoral how-to guide posted here at one time, but it seems to be gone.
Thanks everyone for your help.
2001 E430 4matic
Gone but not Forgotten:
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black