DAB retro fit for cars with the CD 400

[tucpal]

there are a few different options for the way the FM will be received
ether from the standard Ariel on the roof
or from the rear glass this option will be for the cd500 etc
as they use the shark fin for satnav.

so in your case you have the DAB but you get the FM from the roof
you need to check a few things but yes it will be possible to get the FM
from the rear glass
first do you have all the tabs in place on the rear glass
and also check to see if the FM Ariel on the roof is attached to a cable
check them and come back here.