Sorry for the delay my friends! Just took the Sprinter to Tecate Mexico and back! Great trip - only issue on the road was getting a hole in the upper turbo hose (the one that connects directly to the intake manifold). Found that out due to several indicators:
1. Engine load at or near 100% in a headwind - it wasn't like that before, so something was up.
2. Due to engine load, increased engine temperatures
3. Finally, LHM caused by over/under boost.
Inspected the hoses, found the culprit, Amazon Prime to the rescue and now a much better running, cooler engine! Another reason to have some sort of scan tool with these vans - almost a necessity!
Ok, back to the regular scheduled programming:
The violet and Brown should be the coolant pumps power supply.
If the ATC supplies a 12V+ on signal (yellow wire on external harness) then you can just wire that straight to the new D5. Then connect up the aux coolant pump directly to the D5.
Since the ground is common with the van, the purple wire may just go the ATC to trigger the coolant pump?
Some of the colors on the factory unit don't match my d5, however the pin-out is the same, just different colors for some wires.
I just did a quick test, and if I provide 12V to the van harness side of the secondary plug (purple and brown on the heater side), it engages the electric coolant pump AND the blower motor. When I remove the 12V, both turn off.
So, it looks like for the van to function as stock with the 7-day timer, Boost, and Rest, I need to have a +12V on that harness/wire whenever the heater is engaged.