Truck won't start, new battery, lights keep flashing

is turn signal plugged in?
if this is a new bulb try the old one to make sure bulb is not at fault
then need to check the voltage at the connector.
A test light is easiest way from a good ground to each terminal.
if flashing the ground connection could be corroded.
if nothing at connector try the connection at the turn signal switch.

I'll try to help, everytime something gets replaced on these yukons, that circuit needs to be RESET ! Is there a reset button, on your steering wheel, to reset a variety of functions ? , check owners manual, or call your dealer for better advise

Either its a bad new battery or tour altenator isn't charging it. Jumpstart it.

i don't know why but dodge trucks back feed threw the trailer lights and leave light on

Sounds like a weak connection somewhere close to the battery.
The alternator should be able to keep up with the extra electrical load not to mention the storage capacity of the battery. So if the headlights are killing power to the key switch, something is weak between the battery and the key.
Could be a corroded cable, connection at the solenoid, or in the fuse box.

You need a new battery, it is triggering the secuirty system lock out

Gday mate, we had a problem with another late model vehicle similar to that, the bulb that was put into the reverse light was not exact brand product ( ohmage was slightly wrong ) and fowled up computer, correct bulb in place and disconnect battery to reset K A M ( keep alive memory) problem solved, BEWARE, some vehicles cannot have KAM reset except for dealer computer tethered reboot , just try putting the factory correct bulb(s) in and see how it goes, geoff. hope this helped.

The battery won't keep a charge or it won't when it is in the truck? Will the battery keep a charge if you put it on the charger and then unplug it overnight, all of this out of the truck. Then in the morning will it start the truck. If not your battery is bad. If it keeps a charge and will start the truck and then it goes dead after several hours, your truck is drawing a load on the battery. You will have to find what has been left on or shorted out.

Not sure, but I've heard a bad hazard switch can cause strange problems.
Also, if your truck has a body control module that controls interior and lights, that may be your problem, too. Good luck.

I have a 2005 f150 fx4 have replaced battery every winter because when it gets cold the truck won't start, It is not the battery because dash lights, door chimes all work but truck won't even crank (dead). Ford insist that it is the battery.