Commas with multiple prepositional (adverbial) phrases at the end of the sentence on the ground of restrictive/non-restrictive modifier

Prepositional phrases may be attached to either nouns or verbs.

A prepositional phrase attached to a noun will take commas following restrictive/non-restrictive logic (e.g., "her death, in London, was shocking").

However, a prepositional phrase attached to a verb never takes commas, which is why "He died in 1989, in a car accident, in Detroit" is incorrect, because the prepositional phrase "in a car accident" is attached to the verb "died."

Edwin's comment is correct, but I think it should be noted more specifically. When you think of bits like "she died in London in 1990", you can sometimes get really pulled into the syntax, and then when you ask yourself questions like "which part is essential and which part isn't?", you will overthink things and pull it apart like you did above. However, the simple fact is that it all derives from intention. If someone says "she died in London in 1990" aloud, one will tend to think that all of that information is important, essential. However, if you add a pause, a comma, before "in 1990", you might think something different, depending on the context.