Complex sentence: Except for the American market, the J platform was not continued into the 1990s, and not having resources to develop a mid-size car on their own, Isuzu had to resort to forging a short-lived alliance with Subaru.
Split into simpler sentences: Except for the American market, the J platform did not continue into the 1990s. Not having resources to develop a mid-size car on their own, Isuzu forged a short-lived alliance with Subaru.