Seat Heater Unit

WARNING: This page is about a different car, the 2013 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.

Disconnect the seat heater unit connector, and check the continuity between terminals of connector.
Connector & terminal

No. 1 - No. 3:

No. 1 - No. 4:

No. 3 - No. 4:

Fig 1: Checking Continuity Between Terminals Of Connector
Courtesy of SUBARU OF AMERICA, INC.
If no continuity exists, replace the seat heater module with a new part.