Assuming that you are asking about the windshield washer tank, it is under the hood, right behind the battery.
We answer this question here frequently. Same place as the front window washer tank, under the hood behind the battery. It is the same tank.
If it's the same as other Foresters there is only one tank under the hood which feeds both front and back washers.
Doesn't the rear window washer use the same reservoir as the windcreen washers? It does on my 1998 Forester. If I keep the reservoir topped up regularly I am never in want of liquid for washing the rear window. ***I had a 2001 (and now have a 2003) Forester. The front and rear share the same fill tank. The 'pickup' for the rear is set higher so tf the fluid level gets low the rear stops working first.
A Subaru Forester's tank can hold about 60 litres or 16 gallons.
The gas tank on the 2011 Subaru Forester holds 16.9 gal..
The gas tank on the 2005 Subaru Forester holds 15.9 gal..
The gas tank on the 2007 Subaru Forester holds 15.9 gal..
The gas tank on the 2004 Subaru Forester holds 15.9 gal..
The gas tank on the 2009 Subaru Forester holds 16.9 gal..
The gas tank on the 2013 Subaru Forester holds 16.9 gal..
The gas tank on the 2012 Subaru Forester holds 16.9 gal..
The gas tank on the 2003 Subaru Forester holds 15.9 gal..