I've found that it is possible to use the existing rear screen antenna to feed a DAB / FM radio signal to a head unit.
I disconnected all of the wiring from the existing antenna amplifier (apart from the ground feed to and from the heated rear screen of course).
The 12v feed from the head unit was used to power the a new amplifier (A4721.01 about £50 from www.co-star.co.uk
I also purchsed some FAKRA Female Water Blue connectors from the same place so that the input and output signals could be connected to the amplifier.
I connected both rear screen antenna wires together before feeding them into the amplifier.
I used the existing FM/AM cable and I already had another SMB co-ax cable in place for the DAB signal from an earlier failed attempt using an E-bay amplifier that proved to be rubbish!
My head unit is a Pure Highway H260DBi and I'm getting 4 out of 4 bars DAB signal strength on many stations