Tapping into the rear speakers for the line-in to the "High Level" input is perfectly fine. I did it on my old Saturn and never had an issue.
Regarding your warranty, they can only void your warranty if it can be shown that your modification directly caused the failure. If you tap in to some power source behind the dash and fry your instrument cluster, it's on you. If you run your power and ground properly, you have nothing to worry about.
-Roy
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