Back car windows (and most side windows) are tempered for safety as you noted. The windshield is laminated.
This process is usually done by blowing cool air on the hot glass (after forming for example), this create surface stress in the glass, this energy is released when the glass break, fragmenting it.
What you see here are the marks due to the shape of the cool air nozzles. You will notice different pattern on different models due to the securities of the equipment used.
The air nozzles make the cooling slightly inhomogeneous,thus the stress is can have a stronger direction making the glass slightly birefringent. Birefringence is well seen using polarized light and detector Photoelasticity
The light coming from the sky is partially polarized, light from reflection on dielectrics (like glass) also (depending on the angle) which explains your observations