Hi,
since it it secondhand, it will not start in discoverable mode. This only happens on first ever startup.
The buttons used to enter discoverable mode vary from version to version.
You may find something that works for you
here or
here.
2001 9-5 Aero | 2001 9-3 Aero Convertible | 2004 9-5 Aero (with kenwood)