Well, the '89 (automatics?) had a boost cut-out in reverse.
My '90 Anny/auto has brake pedal ignition lockout.
Did all years offer optional front cornering lights?
What year did lighted trunk reflectors begin?
Moonroof switches and map lights differed.
VMM changed.
Traction control went to electric/ABS type.
Windshield washer tank was relocated.
What was first year fuel door went from finger pull to button/solenoid? When did auto-lock doors come into effect? Which years offered JBL Premium sound system?
Auto climate began in '91 or 92?
Has to be a long list, I think...
I have a 1991. So I can confirm that the Automatic Climate Control option was first available in 1991. I also believe that the ECM in my 1991 is like what is in the 1992 and 1993 Thunderbird SCs. I was warned that my spare 1990 ECM may not work with my 1991. I can always tell a 1991 from a 1989 and 1990 based on the passenger-side dash (1989-1990 has some coin storage cubby thing on top of the dash) but the 1991 passenger-side of the dash is smooth.
My 1990 and 1991 have the ignition thing where there is that extra "lock" position in the ignition where you have to push a button to turn the ignition one more click before you can release the key.
My 1990 and 1991 are a pull-finger fuel door. I think I saw a 1992 or 1993 LX with a fuel door that didn't have a finger-open spot (probably the button-open kind)
The lighted reflectors started for the 1992 model year. 1989-1991 Thunderbirds have part of the reflectors go over the license plate (about the size of fingers) but the ones that light up don't have "fingers" that go over the license plate.
My 1991 I know has the JBL sound system, sadly the previous owner messed with the speakers in my 1990 so I don't know for sure there.
I know that 1992-1993 Thunderbirds are basically exactly the same. Except that when I look up pictures on Google (someone please confirm this) I don't see any 1993 Thunderbirds with cornering lamps in the front bumper, but I do find images of 1992 Thunderbirds with cornering lamps in the front bumper. Because its an option, there just might not be very many that have it. I do know that cornering lamps in the front bumper were offered from 1989-1991 and I'm sure for 1992. I'd be surprised if 1993 did not offer them (because it's the same front bumper). Again, someone please confirm this.
I've even been keeping track of map lights and moon roof switches (because I'm officially weird). I parted out a 1990 SC with a moonroof and it had different lights and switches than my 1991 with a moonroof. IT WAS SO COOL, the map lights were NOT in the celling, they were actually in the rear-view mirror (I took it cuzz I had to). My 1991 has the moon roof switch in between two smaller switches that control the map lights (in front of my head). My 1990 (no moon roof) has a single hub thing in the middle of the ceiling (behind my head) that has two switches for the two lights inside. Maybe I should take pictures and save them in a spreadsheet
I wished I took a picture of the 1990 with a moonroof before I gouged out its switch, cut the end of the wire, and broke a small area of the windshield because the bead wouldn't come off.