2003SprinterManuals.zip (2002-2003 NAFTA Sprinter Service Manuals).
02va-parts manual.pdf (2002 NAFTA Sprinter Parts Manual).
My Sprinter had a lot of bad connections in it (mostly at the fuses under the steering column and drivers seat). But I also had issues with my heater control unit (single unit where you select the heat settings and vent configuration).
Dig out the schematics you need from the service manual and see what your find. You could have fuse or relay issues. Many of them are related to oxidation (at least they were on my 2002).