RogerH, These Inertia switches were developed in late 80's and used in many OEM applications on European and US cars (Ford as an example). I dont know what system GM uses. It is triggered by a severe and sudden impact 10+ G's typically. I think these are still in use. The logic is in severe impacts, (depends on the type) engine could remain running while fuel line(s) is ruptured spewing gas, and driver unable to act. The
oil pressure switch type provides a ground or closes the circuit upon engine run (
oil pressure above a set point of a few psi). The inertia type opens the circuit milliseconds after impact. The
oil pressure activated type allows more cranking voltage and current to be available to the starter for first few seconds of crank since it wont turn on fuel pump until engine produces oil psi above a few #'s, an added benefit. NAPA sells a bunch of styles and sizes (of psi activated) I found.
I did some digging and found the OEM switch I am familiar with. Pegasus Racing sells a new OEM type for about $65 with connector pigtail ready to install. Has a nice rubber boot covering the switch body. Breeze automotive sells a used Ford switch with connector for $25. All electric fuel pumps should have either the oil pressure type or inertia type for safety IMHO.