It's cheap and it works!
Replaced the stereo in g/f's car recently and found the aerial from the previous aftermarket stereo linked up with a standard ISO-DIN aerial adaptor lead - hence rubbish FM reception and missing traffic announcements.
@ the previous reviewer F1man - yes space is tight in a lot of cars but Halfords only sell these things too - for £10 a go mind. I don't know of anything else on the market that performs this function, so unless you can design a diode arrangement yourself you're buying one of these!