We have won a judgement against a tenant that did not pay rent. After they moved out we mailed letters to his place of business he wrote on the rental application. However after about 6-9 months of trying we have not received a response. I do not know if it makes a difference but he is in the military. Also the tenant admitted to altering a official document from the military that stated why he could not pay rent on time.
It might be worth your while to hire a collection law firm to properly pursue his assets. Sending such letters will not do you much good.