Need to find out the four digit code

Question

I bought are car with the kdc-x689 already installed, but it wants a four digit code before it will work. I have no way of getting it from the guy I bought it from. Is there another way of finding the code for my unit.i have a panasonic in dash and i bought it n the guy didnt tell me the code is there anything i can do besides try all possible four digit codes???