Small evap leak 2008 DTS 4.6
 

Have changed gas cap ,looking to find fuel tank pressure sensor any help is appreciated

Usually it is right next to where the gas lines and electrical connections enter the tank. I can't seem to find a diagram for it anywhere.

It's right on top of the fuel pump. The good thing it there is an access panel in the inside of the trunk to gain access to the fuel pump and fuel pressure sensor.
Here is the part you will need - http://amzn.to/2ktkaGM
Here is a video on removing the fuel pump -