Locke had many contributions to the Declaration of Independence. Namely, the Natural Rights, the Right to Overthrow a bad Government, and a Government instituted by the people.
The Constitution, however, took mainly Montesquieu's Separation of Powers and Voltaire's Freedoms of Speech, Press, and Religion. Locke's ideas were, however, included in the election process every four years where Americans can choose to either overthrow a bad leader or stick with him/her or his/her party. And the Capitalist economy established by the Constitution allowed for Locke's Natural Right of the Pursuit of Happiness.