Depending on what you have, the speakers are horrible, and speakers are the MOST important part of an audio system. The stock speakers on my passenger model are a pair of 4" in the dash and a pair of 5" in the rear side panels. Both pairs are 15 watt max, with tiny magnets. I installed a pair of 6 1/2 2-ways in the headliner just above and to the rear of the front seats, and will be replacing the 4" dash speakers with better quality ones. Installing speakers in the headliner was pretty easy, as the speaker wires already run right above the windows, and you can tap into them without having to run new wire. Audiophiles will want to replace the factory 20 gauge wire with decent 12 gauge, though. As for mounting speakers in the headliner, make sure the speakers you buy are shallow enough to fit (I think there is 2.5" of mounting depth between the FACE of the headliner and the body sheet metal). Also, some have made a backing plate to attach the speakers to, but I carefully cut the cutouts for a tight fit and used the included sheet metal screws to hold the speaker to the headliner material with no problems. If anything, just replace the stock speakers with better quality ones. There are actually decent sounding 4" speakers, but the mounting depth in the dash is a little shallow, so remove the stock speaker and use it as a guide.
The Sprinter poses some difficult audio problems because of it's cavernous interior. A simple solution would be to put up some quality surface mount speakers up high in the very back, such as Polk Audio surround sound satelite speakers (your radio will run about any 4-8ohm speaker), but they are so far away from the driver that the volume would need to be high, and there would be stereo imaging problems with the delay of the sound coming from both the dash and rear speakers. The ideal speaker configuration is to have the right and left speakers about the same distance from the listener(s), and that's difficult to do in a Sprinter. The location of my headliner speakers is not perfect, but I actually hear the more distant passenger side louder because the speakers are directed facing the interior, and I'm in the "audio cone" of the passenger speaker and not the drivers' side speaker.