Winning or losing a case depends more on the facts of each individual case rather than whether the attorney is appointed or retained. Though you do not mention it I assume that you pled guilty to this offense, and now wish to have a jury trial. The difficulty with the appeal process is that an appeal is not, and cannot be based on whether you think you were innocent, or that the officer lacked probable cause. These types of issues must have been brought up and addressed by the trial court, (the lower court), before you are eligible to raise them on appeal. In short, if you pled to this the only issues you can raise on appeal are issues involving the plea-taking process, (things like whether you were properly advised of your rights or not), and/or the sentencing process. What you are actually talking about pursuing is a motion to withdraw your plea, (assuming one entered), and proceeding to jury trial on the case. Then you would have the opportunity to conduct evidentiary hearings on whether or not the officer had probable cause, etc. You will certainly need to discuss the possibilities of successfully doing this with a retained attorney. Further, there are usually time limits as to how much time can have elapsed before you are no longer able to pursue this. So time is of the essence.