Welcome to the forum
If you have 12VDC to the KeyTv box without the roof antenna attached, I would be checking the coax for a short. It only takes 1 strand of the shielding to short out the coax.
You will have to remove the coax from both ends and then use an Ohm meter to test for continuity between the center and the housing. if there is the cable is bad, most likely the end which can be replaced real easy. if its not then your roof antenna is suspect.
__________________
2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.