Toyota Prius Owner's Manual page 530

5-2. Steps to take in an emergency
Taking out the spare tire
CAUTION

Service plug
Be careful not to hit the service plug when loading or unloading the tool
holder or the spare tire. (P. 45)
530
PRIUS_OM_OM47A19E_(EE)
Loosen the center fastener that
secures the spare tire.