I have a ground strap going from the body to the trunk although I don't believe the trunk is isolated anyway.
World of difference between DC continuity and AC/RF. If you've got an antenna analyser you can check it quite simply by measuring where the resonant frequency where X=0 is without a bonding strap then when you've added one see if the resonant frequency has dropped. It usually does despite good DC continuity. Despite my antenna being on the hatch lid, when I bonded the hood to the body the resonant frequency dropped again indicating even more of an improvement.
You need to be making the ground strap from flattened RG213 braid at a minimum and not wire.