I have a flaming river kill switch in the trunk, but I would like to be able to shut the power off from inside the car as well. What are some ideas that other people are using?

https://www.lokar.com/assets/i...onnect-Lever-Kit.pdf

Generally 2 ways to do it, mechanically or electrically. 1320racer showed you the mechanical way to do it. Electrically you can wire a continuous duty solenoid into the circuit and use it to kill power.