what are the D+, B+ terminals I keep seeing be refered to?
B+ is raw battery power ... usually though a fuse in the Sprinter.
They actually label dozens of points "fused B+", but one source is fuse 3 on the T1N power distribution (PDC) fuse block mounted on the positive battery post. Many of the other "B+" may have passed through more than one fuse, but they're still unswitched.
D+ is "switched" power... in the T1N Sprinter, "D+ Relay One" is pulled in by a signal from the Instrument Cluster.
D+ Relay One then pulls in D+ Relay Two, for additional switched current capacity.
The B+ that the D+ relay switches arises from PDC Fuse 3.
D+ can be thought of as "alternator up and charging"... among the secondary relays that D+ Relay One pulls in are the Daytime Running Lights (DRL) and the MB-installed "isolator" relay which connects the Auxiliary battery to the alternator for on-the-road charging.
In Ye Olden Days, the B+ and D+ were labeled terminals on generators/alternators.
There's a third feed of interest, the "Run-Start" circuit. It's active when the ignition switch is in the "run" position.
(there's also the 4th "accessories" circuit), but i don't think either of those has a "one letter" tag like B+ and D+.
If you have any T1N service manual, you can trace the "B+" and "D+" names in the connector pinouts listings.
They're not frequently labeled on the circuit schematics themselves.
--dick