As ECU said, it's on the side of the driver's seat pedestal.
*Which* fuse it is varies with the options you have in your Sprinter.
On the back of the fuse panel's door is the list of which fuse does what in *your* Sprinter.
(i highly recommend copying that fuse list and saving copies in multiple locations)
For example, here's my side-of-seat fuse list.
For me, the 12v outlet is a 15 amp fuse at position 10.
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Fuse Size Function
1 25 heater booster
2 7.5 auxiliary hot air heater
3 7.5 instrument cluster
4 10 heater booster, term 15
5 25 driver power window
6 15 driver seat heater
7 25 passenger power window
8 10 auto transmission, term 15
9 25 central locking system
10 15 12v outlet socket
11 25 aux heat exchanger
12 15 relay, term D+
13 25 body manufacturer, term 30
14 15 body manufacturer, term 15
15 10 body manufacturer, term D+
16 10 heated exterior mirrors
37 30 heated rear window
38 30 air conditioner auxil fan
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The circuit diagrams given as section 8w of the 2006 service manual (available for free at the address in my signature block) mentions three fuse "panels"
One is Fuse Block 1, which is the panel under the steering wheel.
The second one they mention (Fuse Block 2) does not really exist with the fuse numbers they give... it's a "catch-all" listing of the fuses that *might* be on the side of the driver's seat.
The third location they mention is the Power Distribution Center (PDC) located attached to the starter battery's positive post.
--dick