Cutting power to the 8040 is not a good idea. If you are stuck using the factory fob and want to use the 8040 then you'll have to control it passively.
You could add an aftermarket radar sensor (and shock sensor for that matter) to the factory alarm. They're not as good as the one in the 8040 (which is all it was good for really) but can provide at least modest top-down protection.
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