That's really for turning a damaged vehicle into spare parts rather than resolving whether items stored in the vehicle (like the occupants' backpacks or other items in the cargo space) are damaged or destroyed (my apologies if I wasn't clear enough in my original question). I can use those rules as a guide to house rule something though, combined with a luck roll regarding what has been damaged.
Thanks for the input.
"A hit to the fuel tank or ammunition store in a vehicle can cause it to become permanently destroyed, meaning it cannot be repaired. It can still be scavenged for spare parts, however" (PM, p84).
It's for damaged or destroyed. Clearly the rules take a simplistic view that once a vehicle is destroyed, then so are its contents, except for the ability to scavenge for spares. There's even a detailed bit of flavour text on page 83 about Diaz and Kasia scavenging a wrecked BMP for spares. As you say though, it could be house ruled.