I am a mainly regal guy with some cutlass stuff, I have been building, buying, selling, parting them out and collecting parts for almost 25 years. The rarest option that I have seen is regal rear quarter chrome panels, I just sold the only set I have ever got my hands on for 450 bucks adn really did not want to sell them. They are nearly impossible to find even 20 years ago. Second rarest item which I also have in storage is digital climate control, some turbo buicks got them but I have only seen a couple of stock regals with it and oddly enough mine came out of a regular but loaded cutlass which is the only cutlass I ever found with it. The opera lights on a regal, I have many of them but it is rare to find a set that still actually work. Cornering lights on the regals were rare but more common than these other parts, I have several sets of lights, a good pair of fenders to match and a front wiring harness for them. I did mess up and tossed or sold the column that went to those and there is apparently a sensor on the column. I have those stashed for a future project. Passenger side power seats were a hard one but I owned a limited that had it along with the reclining mechanism which may be even harder to find, I have only seen a couple others. I have no clue what the term is but I have found sets of door panels that have a set of white and red lights in the back lower corner like the rivieras had, I have a set of those in storage. They turned on when the door opened , I suppose so people could see that you have a open door in the dark? Lighted passenger side regal limited dash plaques. Those came 2 ways, with 2 light bulbs or with the same setup that the opera lights came with that had the 110 volt transformer, I can't think of the technical term off of the top of my head. There were regal limited seats with the non shifter console, seen those in both velour and leather, I have had multiple sets of those and had some that I swapped into my first car that was a 78 regal. drivers side lighted vanity mirrors are hard to find and passenger side remote mirrors are hard to find. Astro roof regals are hard to find and a non turbo T top regal is more rare than you would think but they are out there, I seek out T top and astro roof cars, those are the ones I keep for myself if they are worth a crap.