Thank you for the suggestions. However, it looks like I have good continuity to the ground. I used an ohmmeter to measure the resistance between the ground of the flat connector and one of the bolts underneath the plastic threshold for the rear hatch (the only place I could find exposed metal). The meter read 0.9Ω. My meter has a continuity test feature which sounds a tone when there is low resistance. Using that feature, there was a strong, continuous tone. If I should be testing the ground wiring some other way, please let me know.
Is it possible that my T-connector is defective? (
https://www.carid.com/curt/3-wire-t-...mpn-55361.html) The T-connector includes a literal black box that was described as 'a circuit-protected converter'. Might that be 'protecting' the circuit by shutting it down, perhaps in the way a ground fault interrupt does?