on my 90 with roof airs, the romex for the ac runs directly from the breaker box up the wall and down the corner wire chases directly to the area where the ac is mounted and then juts over in the headliner to the ac. so, the hardest part in mine would be in the headliner, but if it isnt secured, you could use the old wire to pull the new one.
havng said all that, there is nothing that in mine would cause the romex to break on one of the lines. it is very strange that it would be open. i assume since you checked at the breaker that it didnt simply come out of the breaker (the copper wire).
mine are not tied to thermostats, so quite simple.
tom
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