So if it's say 1500 kg trucks you get Car 1 issuing 1,875,000 Joules of energy into the other object. Car 2 transfers 7,500,000 Joules of energy into the other car. Since both cars come to a complete stop, the kinetic energy must be transfered into the other vehicle as damage, not movement.

No. 1,875,000 plus 7,500,000 joules (9,375,000 altogether) are converted from kinetic energy to "damage" (sound and friction). But there's no rule that says that all of car 1's energy goes into car 2 (rather than being partially dissipated in car 1 itself), or vice versa. In the zero-momentum frame, the situation is completely symmetrical. Each car will sustain about 4,687,500 joules of damage.

If you'd like, I'll scan and post the pages from my physics book.

If it genuinely concurs that all car 1's energy will leap out into car 2 causing car 1 no ill-effects, and vice versa, then just scan the title page. It'll let us know which authors to avoid

Peter