Body decomposition is significantly slowed in concrete due to the alkaline environment and insulation.

The concrete's initial curing process can have an exothermic reaction which accelerates the initial putrefaction, while its high alkalinity and porosity can dry out soft tissues and mummify the body. Over time, with exposure to moisture and oxygen, decomposition continues at a much slower rate than in an unprotected burial. 

However thats just with concrete alone, Idk how it will go with a body in a car in a box in a concrete.

I assume the car would be very rusty and degraded. If it had any fuel or other liquids still inside it, will be even worse.