I should not be alive, if it wasn't for a reason.
I think, this "was" should be "is" as the speaker is still quite alive and speaking the sentence. Am I incorrect in this interpretation?
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Formally, this is an irrealis conditional and should be expressed with what we may as well call a subjunctive∗:
I should not be alive if it weren’t for a reason.
Informally, however, the ordinary indicative past often—perhaps fifteen or twenty percent of the time— replaces the subjunctive, and nobody notices.
In this particular case, moreover, there’s a complicating factor. There's an idiom, “If it weren't for X, Y would not have happened”.
If it weren’t for Bob we wouldn’t have escaped.
If it weren’t for the recession I’d be retired by now.
This particular idiom has over the last fifty years or so exhibited a stronger—and increasingly stronger—tendency to replace the subjunctive. In the Google nGram below the blue line shows the rate of replacement for this idiom, the red line the ‘baseline’ rate for an ordinary subjunctive.
I don’t think this idiom is in play here—I imagine what the author means is that there must be a reason why he is alive, he would not be alive unless there were some purpose to his life. But the idiom so to speak “infects” his expression of this thought.
Interestingly, as I was researching this answer I came across this blogpost which employs both forms—the subjunctive in the first paragraph and exactly your example at the beginning of the last.
∗ provided John Lawler isn’t looking!