Towing Issues: Trailer light flashing like a christmas tree, f150 ford, start from scratch

Question
Hi Don..
I own a 4x8 fiberglass enclosed trailer.
I used to pull it with a 1997 ford F150 and the wiring from the trailer to the truck were both originally installed from the manufacturer and working in perfect condition.

A month ago I bought a 2005 F150 and when I went to plug in the 4 way flat plug (white,yellow,green,brown/yellow, brown/green)from my trailer to the manufacturer flat plug, here what's happening:

I turn lights on on the truck.....trailer tail lights,side markers, are on ok

I turn on either the left or right turn signal on the truck......all the light on the trailer are flashing

I turn on the hazard blinker....same result ...all the trailer lights are blinking the same way.

I checked the color connection about 100 times making sure the colors are in the correct order.
It's grounded properly from the truck to the trailer.

I am all confused...used to work on a 1997 F150 Ford no problem, 2005 F150 it's all screwed up!

Please help !
Thanks for your time and your patience .

Answer
Despite checking the ground, do it again, most problems are a bad ground.  Don't rely on the hitch, it should be one of the four wires on the plug.

Start from scratch.  check that say just the parking lights on the truck are OK, then check the stop and turn lights.  Make sure they all work without the trailer attached.  Then one by one check the trailer and then plug them together.

Don