Can I retrieve codes manually from a 95 bonneville

If it has OBD1 for a diagnostic system you can get the trouble codes by making the check engine light flash.
If it has OBD2 you'll need a scanner or code reader. Most auto parts stores will check OBD2 for free in hopes of selling you some parts.
You can do an internet search for OBD1 codes and see if the connector plug looks like yours. If it does you can use a paper clip to jumper two connectors and make the warning light flash.
1994 and 1995 were cross-over years when the car companies switched from OBD1 to OBD2 so each model switched at a different time.