Hopefully the code that you are trying to debug is modular and it has a well defined interface. If it is modular, then you can start improving in the code behind the interface boundary. You can add logging, parameter checks, asserts and whatever to help you during debugging. For a large application, error logging with stack traces and gathering of input parameters are a must.

I frequently refactor legacy code to improve the architecture, reduce duplicated code, and bring the code up to newer, proven approaches. Fortunately, I'm in a company that allows this. If you are in a company where everyone is afraid to touch the code, then you are kind of screwed.

Good luck.