5R09A is what you are needing.
I figured out most of that (I got a 5R09A219xxx in my garage, GT, std interior).
Is there a difference in interior trim between assembly plants on any given time period?
There may be, which is why you should include the partial VIN in a question.
With the vintage burgundy exterior, I thought that all assembly plants would be identical with basic palomino interior.
See above. There is also availability. I had a 66 Fastback with red interior (code 25) and it was "two tone".
I guess a better question on my part is what differences do I need to look for to determine the right trim code?
As you initially stated, you did not have a door data tag. Without that, you need to look in "odd" places for clues. It seems that 89 was the "determined code". The chart in the previous post is from a 1989 book "How To Restore Your Mustang" by Larry Dobbs and Donald Farr. There are others.
Jim