Use a tester to measure the resistance between the PTC heater and chassis ground.
Connector & terminal
(B461) No. 1 — Chassis ground:
(B462) No. 1 — Chassis ground:
(B470) No. 1 — Chassis ground:
(B471) No. 2 — Chassis ground:
Is the resistance less than 10 Ω?
Repair or replace the harness.