I Replaced all of my Turn signals and Brake lights with LEDs. They all blink fast, i know its because of the resistance. I bought a new Blinker Relay, (3 Pins) that is supposed to fix the issue. The problem is, i cant find the relay. The one under the hood, isnt close in size and when i remove it, the Blinkers still work. I am assuming that there is another location under the dash or maybe Firewall ???

 

Most signal lite flashers are located under the dash to the right of the steering column.
Some are mounted on a block , some are with a few other relays. The best way I found was to locate it while clicking , then you knew which and where it was. You may need to remove some dash coverings to get to it. Not knowing what year or type vehicle you have, influences the location.

 

2013 E350 Superduty XLT 5.4

 

The flash rate in your vehicle may be controlled by the BCM in which case you may need to insert a resistor into the system to slow down the flash rate.
It is not that it can't be done , but Ford designs the systems to let you know when something is wrong like a bulb being out and when you start changing things you need to make changes to overcome the built in safety features.