For your 2007 Mercury Montego the recommended Hopkins trailer wiring kit is actually the part # 46255 and not the 40475 which I believe you meant the # HM11140475 that fits Ford Freestars.
The # 46255 uses a signal light converter which helps convert the vehicle's taillight wiring to circuits that can work with a trailer whereas the kit you mentioned installing does not. I suspect that's where the problem is. view full answer...
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