In third-person past-tense singular subjunctive, you use were to describe an unreal or imaginary scenario that didn't happen in the past. You might say,
If the other car were moving faster, I would not have had trouble connecting.
But since in your sentence you're just using third-person past-tense singular, non-subjunctive, you should use was, because the example is not a hypothetical or unreal scenario, it is merely unknown.
I'm not sure if it was the other car.
I recommend inspecting the fascinatingly convoluted definition of be, to see how it's used in different tenses and forms: