Antenna cables

Hello, I have the following problem: In my car (Golf III Kombi) I do not have a radio reception, or only occasionally, if the transmitter is not far away. The antenna cable is properly connected to the radio and the antenna is also fine (I tried on another car). I suspect that the antenna cable is broken somewhere between the radio and the roof antenna. Can someone tell me how to find out (measured?) for sure, or how much effort it would be, the cable ne u to move in? Do you have to take off the whole sky? Or other possibility: if I connect a telescopic antenna on the radio and just put the antenna in the car (under the seat or something), do I have reception or shield the coach that? I need a quick solution because I have a long motorway ride in front of me on Friday. Thanks already! Greeting, Forbi