What I'd suggest is first a circuit to detect when the throttle is being pressed, and activate a light or buzzer.
If drive by wire a comparator (op-amp) to monitor the pedal position sensor.
If a throttle cable a normally-closed microswitch mounted to release when the pedal is pushed.
Then simply power that circuit from the brake light voltage, so it activates when both pedals are pressed.
I hope that's clear.
If I could be there I'm sure I could indeed cobble something together from my misc parts bins.
There are safety concerns when taping into that system. If you do that you should be sure the tap connects to a very high value resistor so there is no significant return in the rare event of a component fail.
I would tap into the tack to sense RPM's, and include a delay for not setting off the alarm when the RPMs are still above idle while the engine is reducing throttle after letting off the throttle and the engine is still backing down.
Also, most vehicles have enough noise on the main 12 Volts from the ignition system that you can get a good enough for this application representation of the engine speed just from the small ripple on the 12 Volts.
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Many many years ago ( when vacuum tubes were still in use in some equipment ) someone who worked in repair of hospital electronics told me of a story where a patient was connected to an EKG machine and died. Then a second patient was connected to the same machine and died. Then the hospital figured out there might be a connection in the deaths to the machine and had the electronic department check out the equipment. It turned out that the input of that EKG machine connected to an isolation capacitor, and the other side of that capacitor connected to the high-Voltage grid of a vacuum tube. The input isolation capacitor had failed as a short, and the high-Voltage of the grid circuit was then fed to the leads that were put on the patients, and it killed both of them.
Another case was a defibrillator that was designed with a mirror on the back side of a meter movement that showed the voltage a capacitor was charged up too. The person using the defibrillator would dial in a setting of how high it should charge too, and when the light reflected off the back of the meter movement it was sensed and the charging of the capacitor was stopped. Unfortunately if the lightbulb burned out the charging was not stopped and it charged up way too much and could kill someone. The person who found out about that problem was afraid to turn it in because they feared the hospital would get mad because they would have to scrap all there defibrillators. I told them to get a blank post-card from the post-office and send in an anonymous card to the FDA explaining the item and the problem. That defibrillator was quickly recalled and removed from use everywhere.
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When you design electronic systems that have anything to do with critical items, you have to consider the posable fail modes.
I doubt that drive by wire throttle uses only one simple variable resistor that someone could tap into. If it uses variable resistors, it probably uses several and compares them to detect failures and fail safe, and that would probably all be in a sealed unit you could not access. It also may use some kind of binary gray code arrangement, and even if it uses that it most likely would be redundant with comparisons and fail safe.
I would stay away from taping into that end of what was going on, and look at the tack somehow.
