I did this modification in my 1.3. You run a flexable arial up the side of the car in the diagonal strut between the door glass and the windscreen and you use the screw for the sunvisor to hold it in place.
The signal you get is really good and you don't have to worry about an aerial outside the car getting bent or broken or whatever.
However I had to do it by using an FM aerial I think from the Sony Centre and welding a socket onto it with only partial sucess.
Can you buy flexable aerials from somewhere?