vpwr fuse keeps blowing

[sst1972]

2008 ford f-350 with 6.4l diesel
has any one ever had this problem and know how to fix it?
was driving around today and truck quit. just like shut it off. checked fuses and found the 20 amp fuse for the vpwr was blown. replaced and blew again. tried several times same thing. put a wire in fuse holder and it came red hot. anyone know what i should be looking for and where?