New ride new ideas

well a couple days ago my devillejust turned 126,500 miles over. I had it for more than a year and put about 7k on it. Well i sold it and now im in the process of acquiring a 1993 Cadillac Fleetwood Deville. I only looked at it and i think im guna have some troubles installing a new system. I dont know why cadillacs have to have such big bulky headunits but they do. I dont think anything will fit in there unless you put like custom made fabricated brackets to support it but even then itd look like crap cuz there would be gaping holes... as far as the speakers go... i cant find em. I believe theres gota be 6x9s behind the rear seat under the rear deck lid like in my old 94 i wana buy a better system than what i had. The sonys were put in custom jobed they had to have mounting brackets tack welded on to fit properly in the rear deck lid (this was what i was told) and the front door speakers same thing they cut holes in the kick panels and put em in there. I didnt take the car to a pro shop i didnt have the money and plus my belts and hoses needed to be changed so i took it down to the local garage. They also repaired the window motors cuz the windows went up and down slow. im thinking about just bringing the new car into bestbuy or something and have them install a new system piece by piece. if the people at the garage didnt have to cut holes into the kick panels id take all the components out and put them into the new cadi. Any ideas on Headunits, speakers, amps anything?