I'm guessing that you know the answer. The time to have raised your, potentially quite valid, defenses would have been at trial. Literally, you had a date in court and you misssed it. We all have constitutional rights, and we all have the choice to waive our rights. This is what you have done. Barring extraordinarily unique and exceptional circumstances, you will need to comply with any sentencing. Consult with a criminal defense attorney.
This answer should not be construed as actual legal advice, which can only be provided in an office consultation by an attorney in your jurisdiction knowledgeable with respect to the area of law in which your concern lies.
Generally, a defendant has to move the court for relieve from judgment under Ch. 800 and/or section 806.07 within 6 months, and must show excusable neglect or some good reason for doing so. Example, the person was hospitalized or deployed. There may be grounds for bringing a motion to reopen after 6 months if the circumstances are compelling enough. It is, I agree, very unlikely after this amount of time. However, some judges are frankly more generous than others about this. You should cotact a good traffic attorney in your area to discuss the specifics.
Not legal advice. I am not your attorney.
I will respectfully disagree with the second answer in part. Also, I would disregard the first answer entirely, as the attorney is not licensed to practice in WI.
First, I believe this was a 1st offense/civil OWI. I believe that because you can only be defaulted for a civil OWI. For criminal OWIs, if you do not appear, a warrant is issued for your arrest. For a civil OWI, 806.07 allows you to reopen the judgment against you, if certain reasons apply to your case. Certain reasons to reopen under 806.07 require those reasons to be brought within a year. Others, such as any reason justifying relief (the catch-all) do not. To my knowledge, there is no 6-month limitation/demarcation anywhere in 806.07. Although, perhaps there is an informal presumption in favor of reopening within 6 months and an informal presumption disfavoring reopening after 6 months.
You may have a valid basis to reopen now, but the sooner you file, the better your chances.
Consult directly with a local, experienced OWI defense attorney immediately.