Testing mobile phone anntenae issues

I thought I knew what I was doing but it seems not!......

The issue is that we have a suspect antennae connection between the antennae on the bull bar through to the phone..as no increase in signal when in a weak area when in the past there has been

So I placed a multi meter on the fme connector inside the vehicle and got an audio signal with the probes on the centre pin and the outside shield...........so I assumed a short somewhere..........as I assumed that there should be no connection between the two

I know for sure that in performing the same test with a UHF or HF that there should
be isolation between the centre pin and the external braid sheath and so assumed the same would apply with a mobile phone cable?

So in tests so far ....testing an unconnected patch lead....no continuity between the centre pin and braid......same applies to the lead from the spring base with or without the patch lead attached....all good .....BUT connect the whip and there is connectivity between the pin and braid

Disconnect the whip...all silent again...check the whip itself and there is audio between the pin and the thread base.........

Tested another identical set up that works perfectly and once again the same test results of continuity between the points with the whip and not without

So where I am getting this wrong ?.......surely there must be no continuity between the centre pin and the braid for a mobile as the must be in a two radio set up????