No trade-in nothing changes, you pay on negotiated price of vehicle before factory rebates. Would only affect purchases with trade-ins.
Now if you negotiate $60,000 (not sticker) on a new truck and trade in a $30,000 truck you pay sales taxes on $60,000 (6%=$3,600). If the law passes you pay sales tax on the difference of $30,000 ($60,00-$30,000 (trade-in) =$1,800). The state of Michigan loses $1,800 and you save $1,800.
Note-Rebates do not count as they are considered your money and cannot be used to reduce taxes.