How hard would it be to add to a car that didn't have it factory? Would I be better off just buying an aftermarket kit?

If you have the factory wiring diags, it is not a difficult (no more than installing an aftermarket) job. I did it on my '91 deville. keyless box donor was a 1991 fleetwood. I already had the remotes from my '91 fleetwood. If you have the remotes and can find a keyless box from the wreckers it can be done. Just make sure to cut the wires long from the donor car as non-keyless cars are not pre-wired.

Chris

Speaking of keyless entry my 91 fleetwood has the security package with keyless entry but I have no remotes. Ignorant as I am I tried to program an old remote from a 95 caviler by jumping the wires from the ALDL connector. When I did jump the wires the doors unlock then lock and the trunk pops and i press the buttons on the remote but it didnt program the remote i had so i tried it again and again and on the fourth time i jumped the wires it would not lock and unlock and pop the trunk. Now my locks still work but they seem posessed by sometimes locking and unlocking while im driving and also they unlock by themselves while it is parked.
Do you think i just burnt the keyless module or what?

I was looking for replacement keyless entry remotes not too long ago and I found a site that had them for $49.99 for my 96 Deville.

I had them delivered in less than a week and the programming instructions only took maybe 10 seconds?

Anyways, here's a link to the site:
https://www.ThePartsYouNeed.com

Just click the Cadillac category, or type DEVILLE in their search box.

For the 91-93 FWD Cadillacs, GM used TRW remotes. Three button with the trunk button below the lock & unlock buttons. These remotes were also used on Buick regals & some chev trucks. Most oldsmobile & larger buicks use another manufacturer, although the remote is very similar. Remotes from mid '90s dodge intrepids will also program to the Caddy module. I don't believe that the cavalier remote is compatible. I have a couple of spare buick & dodge remotes. possibly 1 cadillac remote.

Regards,

Chris

This kind?

http://i13.photobucket.com/albums/a290/thepartsyouneed/Cadillac-ABO0104T.jpg

That ThePartsYouNeed site has them listed for the early Deville and Seville.

So where is the keyless entry module located? Anyone got a pic of what it looks like? Also anyone parting a deville with keyless? I am looking for a module.