Wait, I was just marked right for the same translation. There's a big dif between I am your witness and your witness is mine.
You could say ""ben senin şahidinim." This is just an alternative, yet equally valid way to word that. The ending on "ben" might be throwing you off. It looks like "benim" as in "my," bu it is actually "ben + -(y)Im" which is "I + 1st person singular copula (like am)"
That sounds extremely Denglish/German-English and/or archaic in English. :) Word order tends to not be very flexible in English.