Being as Michael won't respond (Quite possibley because he doesn't want to divulge any info for free) I'll throw in my two cents.
NOTE: I have never delt with a factory keyless entry system, so I may be wrong. I highly doubt, nor do I really see how, that it would be differnet from aftermarket systems, though. So wire away!
It really is not too difficult at all to add remote start feature in place of the Gas Door feature (OEM Keyless entry) The most important item is, natrually, a remote start system. Really the only key is to purchase one which can be activated by an accessory channel from a second module of some sorts (Generally speaking, an aftermarket Keyless entry system with a spare channel). In order to activate the Remote Start module with the accessory Lead, all that needs to be done is to apply current to it. Generally a positive current, I beleive.
I have had good luck with Bulldog Security. I just hate their remotes with a passion. So hidious. But hey, you won't be need that anyway!
Their bearbone system will work just fine. being as it has the lead needed to run it from another module. They have some pretty sexy systems, though. The high end ones will start you car automatically in order to keep the cabin/engine warm, or Cold as the case would be in the summer.
Being as Cadillacs use VATS, you will also need to purchase a bypass module. Once again, Bulldog securiy has a really nice one, along with a really cheap one that will get the job done. Links posted below.
Part 791 (highly recommended) - http://www.bulldogsecurity.com/bdnew/accessorynew6.htm
Unfornatly, it appears Bulldog Security no longer makes a lone Remote Start Module. They all include keyless entry now. I just saw that as I was looking for the link to give you guys. Any brand will work, though. You just need to make sure it can be activated by an accessory channel.
But, on we go.
First off, you need to install the Remote Start module just as your normally would. Bring it all the way to the stage where it functions just fine on its own.
Next you will need to locate the Factory keyless entry system. From the one post I have read, it would appear they are located in the trunk. I would imagine the general location would be standardized. But, once you hunt it down, you need to figure out which wire is used to control the Gas Door. It would probably be easiest to trace the wire back from the relay. If this is not possible then you could easily enough use an IC safe circuit tester. Using the circuit tester, you would just need to probe the wires on the Keyless Entry module and find the wire that is applied power when, and only when, you pop the fuel door via the remote.
Once you have found the wire that contols the remote gas door pop,a nd are certain it's the right one, go ahead and cut it. More than likely there are two relys, one for the interior button, and one for the remote. If this is the case then joy. If this is not the case, then be sure to only disturb the wire going to the relay which comes from the Keyless Entry Module. Otherwise you may not be able to use your interior button. What fun that would be. (If you are just useing the wire straight out of the Keyless Module, then ignore what I just said.)
Once you have found the correct wire and stole it from the gas door, you need to find out what its polarity is when operated. I would imagine it would be positive, though. Due to the Chassis being Ground. Once you have figured out it's polarity you need to read through the manual on your Remote start module. Find out which polarity it must receive in order to activate via the accessory lead. If the two polarities match (Keyless Entry Module Put's out + and Remote Start requires +...or the complete opposite) then run the two wires together. Then test.
If the polarity's do not match up, say the Remote Start requires a negative charge, and the keyless entry puts out a positive charge, then you will need to purcahse a relay in order to cause the Keyless entry system to issue a negative charge. If the Gas door did indeed use to seperate relays for interior and remote switch, then you could just rob the relay originally used by the remote. If any of you need to do this but don't know how then feel free to send me a message. I would explain it now, but I can't remember off the top of my head. Once you have the correct the opposing polarities, though. wire then together and test.
That is really all that is required. If for some reason it doesn't work then go again and message me with any details you can furnish. Maybe it could bring forth a memmory I forgot the explain. But, I beleive that is all.
This information also all applies to the Remote trunk pop feature. So if you rarely use your trunk, or can live without the remote feature, then you could use it to operate a differnet device. Such as a Window Roll Up module.
I hope my directions were all clear and you all have good luck with this.