From new on a 156, I'm pretty sure the aerial is in the rear screen for a pre facelift car (pants), it is on the roof for a facelift car. I have a facelift car and the way to tell would be to look on the rear screen and see if there are 2 distinct set of 'elements', one for aerial, one for defrosting - sadly, my car is in the menders and I can't!
In any case, if you get a DAB radio there are 2 aerial sockets on the back of the headunit, one for the DAB signal, one for FM. You plug the existing aerial into the FM socket and you have to rig up a new aerial for the DAB part of the radio.
I had a DAB radio and I found that the little stick-on electronic windscreen aerial didn't work - it may have been a fault or it may have been bad reception in my area. Maybe it would work for you? In any event, a proper exterior aerial will be far superior and stop dropouts, as well as giving you a greater range of stations.
Audio-wise, I found the DAB to be better than FM, especially on speech-radio - I guess mainly because of the shortcomings of the FM aerial in the 156. Now that I have a facelift car with the roof aerial as standard, even that doesn't have great reception! With a good DAB aerial, you will definitely enjoy Radio5 Live a lot more than you do now.
I sold my DAB radio after a while - why? Because there is no Traffic Information Interrupt on DAB. Can you believe it? You don't get local traffic reports, they didn't design it into the DAB protocols because it was rushed out and they couldn't get international agreement on how to do it - doh! It worked fine on the FM side of my unit but it wasn't there on the DAB side.
DAB II is coming out over the next couple of years, as I understand it - bit by bit, DAB I will become redundant. Who knows, maybe this time they will put the flaming traffic information in, pretty important in a car radio, don'tcha think?