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I find a square grey connector under the dash, next to a black flat pin connector. I understand the flat connector is for emission testing. Is this correct? If the square grey is for trailer brakes, who makes a connector to fit it?
I haven't looked yet, but, is there a plug for a 4 pin in the back?
If that grey connector is 6-pin with only 4 wires used, that should be it for a brake controller. Just about any controller manufacturer can supply you with the correct plug-in harness (I really like the Tekonsha Prodigy - not the cheapest but an excellent controller).
Expy/F150/F250/F350--all have the same 6 pin female connector up under the dash. It should be to right of steering wheel, may be pinned to a bracket. You will need a pigtail that matches the connector (usually supplied with the tow package in the glove compartment) with 4 wires you have to splice to the controller wires. One of the Ford part no's for the pigtail is 2C3T/15A416/AE. I believe the 15A416/AE is common to all trucks, the first 4 may be what vehicle it comes with.
Lots of brake controllers on the market--Prodigy gets the most kudos on this forum.