Help with writ of garnishment against tenant in MD?

I am about to file a writ of garnishment of wages against my tenant, since I have a court order, properly executed eviction, etc. However, I have 2 questions:
1) on the form, I am supposed to put the last known address of this guy. Well, he moved without telling me where he was going nor a working phone number. What address am I to put?
2) Other than filing for bankruptcy or proving that he can’t survive on his current salary (which I doubt, because he is a store manager, and it’s not like I’m taking a huge amount from him per paycheck), how else can he avoid paying me? I know he has 30 days from the initial writ to challenge, but after that? And on what grounds can he challenge the writ, anyway? He lost the initial decision in court anyway, which led to his eviction, and this is just the final step.