The block heater information in the 2007 Service Manual is for a T1N. It mentions a "core plug hole" in the block. There isn't a "core plug hole" (at least that I've found) on the OM642. If you find one, photograph it and describe the ID and theads, please.
In the pursuit of my own (not yours, my own) knowledge, I ordered some of the training material from Techauthority.com.
I ordered some Sprinter technical training information from Techauthority.com: "VB Sprinter Service and Maintenance Introduction."
The part number is 81-699-05064A.
I
do not recommend that anyone purchase it for anything other than your own curiosity.
The book does, however, mention a block heater, and this Chrysler publication's text lines up with what our Finnish collegue said
here about block heaters being in a coolant line.
I've attached images of the Book cover, the table of contents indicating "Block Heater .... page 106" and pages 106, 107, 108 and 109.
(No, there is no picture of the diesel OM642 in this section of the book; only the gas engine, M272)
Observant readers will notice that there's a gap in the table of contents between 106 and 109.
Those pages do not contain what you're looking for, but I'm posting them because I have three more images available in this post and I figure you have the right to see for yourself.
-Jon