My chinese WD6050 uses one of those. ...
But why?
Possibly on other makes you are combining AM and FM, but not on a MK V.
On Mk Vs with twin analog antennas,
one antenna lead is an AM/FM feed.
The other is FM only and is specifically filtered of AM signal.
Both are already amplified at the antenna in the rear hatch.
Amplifying at the other end of the cable ampifies the line noise too and negates the advantage.
Connecting them together should make FM reception worse,
as that will create FM multipath distortion, as
that is not how they are intended to be used.
On radios that do not offer FM diversity,
simply ignore the FM only feed and you will get a better result.