First off what year is your Caddi? I have a 1993 Eldorado that i too replaced the factory stereo with a Alpine CDA-9811. Unfortunately there is only two options you have when replacing that stereo. First off if you have 1993 and up ( only this starting year because that is what i have) then you can go to
www.installer.com and they have the harness you need,88.95, a bit pricey but it's there.I do believe they have the trim plate you need too. It also has a little pocket underneath the deck so you can store small objects. A pack of smokes, remote for stereo and a pack of gum is what i can fit.The second route you can go is to just rewire the whole system..which alot of stereo shops will tell you that you have to do if they are not familiar with the new products. Now on to the bad part. The harness intergrates the factory amp and bypasses the tuner which you already know are behind the power antenna in the trunk. The deck i picked has a sub out...so if i wanted to add a sub i would be ok, however, if you wanted to add another amp to the front or rear..i believe that you would have to re wire the whole system.
Hope this helps you out..