I didn't add it
I also have a low optioned Caprice that over the years have added a lot of the factory options from B-Body donors....Power Seats, side mirrors with Heat, Cortesy Door lights and soon to add lighted visor, Rearview mirror and coat hooks.
I picked up the entire twilight setup quite a while ago and was going to add it but ended up selling the setup to another member. The instructions I had showed a Gray Plug (taped off) up in the dash area around the cluster for this to be added. The problem is my low optioned Caprice, did not have this built into the wiring Harness. Neither did the other stuff that I have added. I had to make up my own harnesses for everything since they weren't there hiding somewhere. After not having this feature for 15 yrs, I decided it wasn't worth my time to investagate it more.
Post it up if you decide to do this.