The TPP is horrible in a number of ways. It creates so-called free trade between the countries in a number of areas, including automobiles. Unfortunately, if you know anything about the markets you know that even while the Japanese may not place tariffs on automotive products from the US, their market is absolutely closed to US product through a number of other legal but fairly immoral actions.
To top it off, the Japanese are even WORSE at currency manipulations than Americans. As of this second, Japan enjoys an $8000 imbalance between autos made over there versus what we can make them over here, specifically because of their intervention in the currency market.
Free trade doesn't work when countries can play games, dumping products and using massive government subsidies to drive people out of the market. The Automotive industry is virtually the last bastion of American manufacturing, and supports a huge proportion of what remains of the American middle class.
This is a BAD partnership. Oppose it.