You could try 0000 which is the anti-theft setting before owner input but if the previous owner put in his/her own code, the radio would have to be sent to Clarion to have it "cleared". This assumes you are in N America because you didn't add a location. Batteries CAN be changed without voltage loss but most shops don't bother.
The initial security code readout will show 0000--- I would try 1111 as you did already, -if that doesn't work there is likely another code entered, which cant be reset. You cant cancel the existing code without inputing it first. Maybe try finding out who the previous owner was and contacting them??