Will every car seat work in every car?

No.  The issue of compatibility is a leading concern with child safety seats.  Motor vehicles are made for adults.  Child safety seats are an adaptation to the vehicle design.  Compatibility issues between the vehicle, the child safety seat, the child and the seat belt system must be overcome for a safe and secure fit of the child safety seat.  While the LATCH system helps with these issues, it is not a cure-all.  Two-door vehicles and sport models offer the greatest challenges to a safe car seat installation.