If I understand what you're trying to do, you're installing the aftermarket diesel-powered heater and you're trying to install it to also extract heat from the OE heater core in order to heat the 300TD in case you're going to camp/sleep in the wagon? Is that correct?

Page 15 describes the function, so you're looking to provide an auxiliary path of power to the blower?

I'd recommend caution since the blower motor is a high-current device and can draw excess current through the fuse box. There's an auxiliary fuse-box mod that was done by M-B to reduce the likelihood of melting the fuse box @ fuse 8, I believe.

Also, routing of airflow and clean, fresh air intake into the cabin via the OE ducting should be carefully planned and checked if your diesel powered block heater exhausts within the engine bay but you're using the OE heater core.

The link provides the schematics for airflow control, so that may be useful: http://www.peachparts.com/shopforum/general-information/295241-scans-airflow-w123-climate-control.html