"They that can give up essential liberty to obtain a little temporary safety, deserve neither liberty or safety." Benjamin Franklin - 1759
The closest covenant I could find in the Wisconsin State Constitution that could be used to declare the state seat belt law unconstitutional is:
Equality; inherent rights.
SECTION 1. [As amended Nov. 1982 and April 1986]
All people are born equally free and independent, and have certain inherent rights; among these are life, liberty and the pursuit of happiness; to secure these rights, governments are instituted, deriving their just powers from the consent of the governed. [1979 J.R. 36, 1981 J.R. 29, vote Nov. 1982; 1983 J.R. 40, 1985 J.R. 21, vote April 1986]
The "inherent right...liberty" would imply each person shall decide whether to wear a seat belt or not.
The constitutional basis for stopping the federales at their blackmail maneuver is Amendment IX:
The enumeration in the Constitution of certain rights SHALL NOT be construed to DENY OR DISPARAGE others (rights) retained by the people.
The people have the retained right to decide to wear a seat belt or not.