Good luck getting a cb antenna to work correctly on a 387. I have spent close to $250 in the last week trying to get mine to work like i want but i have said to hell with it and given up! The factory antenna locations suck,even with different antennas because of the poor ground and #$%^&* coax so forget low SWR there and i cant get the SWR down with a antenna mounted on the back of the cab because of the reflect from the cab and trailer. No matter where i mount it the SWR is still over 3.0.
This mounting location is the worst idea that Pertbuilt....and Kenworth ever thought of!
They make an antenna mount just for these trucks but unless you have a grab handle mounted on the inside of the cab faring you cant use it,unless you go and buy a grab handle yourself.