From what I have come to understand speakers in the rear tray do not serve any acoustic purpose. one of those reasons are, no 6X9 or oval speaker is made to perform in IB conditions, therefore limiting there performance and throwing the acoustic placement frame out of the window. Another reason being the placement of the tweeter and midbass is such that differentiation in high's/mids/lows is hard to come by or barely there. Another issue is the power handling the oval can take slightly more power and therefore one needs to make an adjustment with the FADER option to make the sound balanced between the front and the rear stage and here comes the problem the person sitting at the rear seat only gets to hear the rears and not the front comps as the ovals are place right next to his ear level. So overall if you look at it the parcel tray is amongst the least acoustically pleasant places for fitting the speakers not matter if its a hatchback/saloon. Ideally the most acoustic positioning of speakers is in the doors. Since they can be made to be used as enclosures for the speakers giving them a environment in which the speakers are made to perform. So even if you want to derive the best output with least input one can simply opt for comps at the front and co-ax's at the rear with the OEM hu (delphi/blaupunkt), keeping in mind the fact that the comps are not power hungry. As for the vibrations u faced in the door that could be cause of either the speaker not being mounted properly, MDF rings not being used, MDF rings if used are thinner than the required width, screws not properly fitted, the way damping is done, clips in the door pads not locked in properly. Alot of these could contribute to the vibrations. So thinking that these stock locations aren't acoustic just because of some incomplete/improper fitting is not just. Now about what the blaupunkt guy had told you, punto and linea's are considered pretty decent cars for ICEing, owing to the fact the size of the door and the metal used, it forms a very good and efficient enclosure. The only drawback as it seems to me and a few others is the stock location of the tweeter, its a little low for some peoples taste. Also a plus point in fiat's is it packed so tight that there is very less room for air to escape making the car a very efficient enclosure and with additional damping things keep getting sweeter.