This is the best idea, with the boot open pull the headling down at the back just enough to undo the bolt which wont be hard, then cut the wire off the aerial as close to the base as possible to give you the most amount of cable. Buy a male and female tv aerial adaptor and attach your new aerial leaving the adaptor in the roof space, Job Done
Failing that drill a hole in it, bend an old coat hanger into an amusing shape and stick it in the hole!