You've crossed subjects between clauses, Prof. El subjectivo numero uno of that particular sentence/clause is 'the Infiniti that I'm trading in', not 'I' The car is 19. I, OTOH, "will not recover" is the subject/verb of clause ni in that run-on sentence. (Mea culpa)
Me has got 3 more decades on my odometer than my car does.
Also, it isn't an "and", it be a Tesla THEN a Polestar.
I refer you to Rule 11 of Strunk & White's
Elements of Style, "A participial phrase a the beginning of a sentence must refer to the grammatical subject."
You have two sentences joined by a conjunction: "A contributing factor to this decision was that I had to replace the battery in the Infiniti that I'm trading in," and "at 19 years old, the cost of any repairs I do will not be recovered in trade-in value."
Thus, the participial phrase "at 19 years old" must refer to the subject which is "the cost of any repairs." So, technically speaking, I had it wrong, too, and the cost of the repairs would be 19 years old. That seeming unlikely to be your intention, the next available subject would be the "I" in "I do" which is where I attempted to point out what I found humorous.
In no way of analyzing this sentence is "the Infiniti" used as a subject. "The Infiniti that I'm trading in" is an indirect object. (The direct object is "the battery.") These are all parts of the sentence preceding the conjunctive and, therefore, the adjectival phrase cannot be considered as modifying them.
Thus, referring to Rule 20 (ibid), "Confusion and ambiguity result when words are badly placed."
But, hell, everybody knew what you meant, right? As that is the fundamental purpose of language, you were fine and I'm just being pedantic in a rather lame attempt at humor.