When to install car seat?

Sydney

I’m a STM and for the life of me can’t remember when we installed the car seat! I know the risk if you get in an accident, the car seat would need replacing.

We have a 5 yo (my step son, not with us every day) who will be moved to the third row and a 1.5 yo in the other captain seat.

I’m 37 weeks, first baby came at 38w5d. Should I install soon? I can’t decide 😂