Show how much time you're spending on working around broken things. Convert that to dollars and missed opportunities, slower execution, etc. And you should have

Show how much time you're spending on working around broken things. Convert that to dollars and missed opportunities, slower execution, etc. And you should have a compelling case for focusing on refactoring. Without management buy-in, this will be impossible... But yeah, as already said here, small steps. Do this for small areas of the code and stay away from the feared major rewrite.