It might be:
* A separate, smaller tank: Look for a smaller tank near the larger front washer fluid tank, often with a cap clearly marked "Rear Washer Fluid" or a similar indication.
* Part of the main tank: Some models integrated the rear washer fluid into a compartment of the main tank. Check for a second filler opening on the main reservoir.
The filler cap will usually be similar to the front washer fluid cap—possibly color-coded, but not always. Look carefully, and if you're still unsure, consult your owner's manual. The owner's manual will have diagrams showing the location of all fluid reservoirs.