I think unless you have some pretty extensive electronics
stuff in your tool kit, you will have to buy a shorter shielded co-axial cable with BNC connectors on the end... or suck up the extra weight and coil it.
If you cut the shielded co-axial cable and try to connect the conductors together, you will end up causing signal loss in the cable. The outer conductor needs to be a perfect cylinder for the co-axial's lossless properties to work.
If you're in the USA, Radio
Shack will probably have a shorter co-axial cable with BNCs.... although if I remember correclty, the cable on the Garmin
is not serviceable at the antenna