The Curt T-Connector # C56015 is designed to be an easy installation on your 2008 Mercury Sable so since you made sure that the tees with the yellow wire was on the driver's side the only other things that could have been messed up is if you either didn't connect your Sable tail light wiring into the tee or if you didn't get a sufficient ground for this harness.
It's also possible that this wiring harness is malfunctioning. You can determine if this is the case by testing the wiring that... view full answer...
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