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BCM reprogramming, possible telescoping steering wheel if you have it, of course the seats as you know and the door control module switches. I'm not sure about anything else but there's a lot involved with the memory such as the HUD, HVAC and so on. I'm sure someone has done it therefore better advice than mine. Personally, I would have bought it with it up front rather than trying to add it later. I did exactly that with my '07. Ordered it upfront with everything I could put in it so I wouldn't be adding it later but yeah, it cost me a lot, just not as much as trying to add it later and on the C-6's, that's some serious issues. I would guess you could do it, but this will give you a TTT that someone might see who has done it. Good luck on the project though!
I've added all options to my C5 except active handling. It all started when I picked up a pair of memory seats for the car. The module underneath the driver seat controls all the memory, no bcm programming needed. Get the memory 1 and 2 switch and it will control everything properly. If you want memory in the mirrors you will need to get the memory mirrors and modules and harnesses that go inside the doors. If you get a telescopic steering column you will need a harness to connect to the module underneath the seat. The harness is not available from the dealer and will have to be custom made or taken off a main harness from a wrecked car. Without the harness the telescopic feature will still work but no memory. If you do a hud you will need a late model 99 and up speedometer cluster with the pin connector on top and the controls for it on the left. 2001 and up electromatic inside mirror isn't that hard to swap out. My car came with dual climate control so memory remembers the temp setting too. Also my car came with the F45 suspension and converted to the new F55 magnetic ride. All the options I added to my car came out of wrecked C5's.
Maybe it's me, but I have the distinct impression that the memory functions of our fine GM product are not a strong point of the machine.
Expensive, prone to failure, don't add it. I wish mine would go away. At least stop the guy that sneaks into my car at night and changes the settings. If I ever catch the SOB, he's in real trouble.