Part of the reason for this mod is that I didn't want to use one of those ugly stick on DAB antennas
The heated screen elements are all the antennas needed. In fact DAB signals are horizontally polarised, so the rear screen antenna is ideal!
The reason that an amplifier was used in the first place is to boost the FM/AM signals, which are transmitted by vertical antennas, to make up for the inherent losses when trying to receive a signal with a cross polarised receive antenna.
If I was to do this mod from scratch, I would do the following:
For the AM/FM signal output from the new amplifier to the radio Google 'Female FAKRA to Male Din 5m cable'.
For the DAB signal output from the new amplifier to the radio Google 'Female FAKRA to SMB 5m cable'.
Run these two cables from the new amplifier down the nearside of the car behind the left rear speaker, under the carpet by the passenger door and over the footwell to the radio.
Google 'Female FAKRA cable repair kit'. This is FAKRA connector to a flying coax lead.
Solder the centre wire of this coax (make sure to not connect the coax outer screen to anything) to the ends of cables 2 and 3 in the attached picture. This connects together the two AM and FM antennas built into the rear screen and their received signals are now used as the input to the new radio amplifier.
I think that this is a far better way to do it than my bit of a bodge up!