Carter,
Here's what I would do with my car, what do you think?
This is a solution to $ 10 - $ 15
Run the reactor only if the transmission is in "D" (with a relay). So, no reactor if the transmission is in "N", "P" or "R".
In addition, shut down the reactor if I press the brake for 5 seconds (using the bkake light, a capacitor and a relay)
Is what you will see a gain, a potential loss or any problems?
thank you