I thought the plastic piece on the rear reflectors didn't connect all the way across until 93? Or is it vice versa? I'm ashamed to ask that because I don't even remember what my 91 looks like it's been sitting under a cover so long
If you are talking about the decklid reflectors, 1989-1991 Thunderbirds are "one piece" (in that both are connected in the back behind the license plate) but the reflector parts themselves look like two separate pieces with a seam between them (the red plastic part of the reflector doesn't actually connect the driver and passenger side). If you try to buy a driver's side reflector, you're going to end up buying both driver's side and passenger side at the same time because they are attached to each other as one big piece. One could use a reciprocating saw and cut the black plastic in the back where the license plate goes and use a driver's side from one and a passenger side reflector from another if you wanted (maybe if you are trying to save the better half from two sets).
1992-1997 Thunderbirds have reflectors/lights and are two totally separate pieces. They do not have a tiny piece of reflector going above the license plate (like early ones). They also appear to have a slight "bulge" or "curve" in the middle rather than being totally flat vertically. These are much easier to order in the mail.
I converted my 1990 Thunderbird to using 1997 LX taillights/trunklid & refletors because I liked the fact that 1992-1997 trunklids glow when the headlights are on and I liked the spoiler that came off the trunklid. FYI: The 1997 Thunderbird LX I parted out did not have black paint on the bottom inch of the taillights/trunklid reflector. Supercoupes from 1992-1995 did not have black paint at the bottom but LX models did have black paint (and probably any other non-Supercoupe from the time). So if you are looking for 1992-1995 SC taillights, you can also look for 1997 Thunderbird LX taillights, maybe even 1996 Thunderbirds too.
I'm only sharing because I met a Supercouper the other day who didn't know you could get 1992-1995 SC-style taillights (no black paint) from 1997 Thunderbirds and maybe even 1996 Thunderbirds.