The back of the trackpad has a Broadcom BCM5976A0K chip on it as well as a Cypress CY8C24x94-24L PSoC chip.

The CY PSoC chip is likely responsible for the multi-touch capabilities on the trackpad as it has touch capability and has been found on many MacBook trackpads.

The board is cleverly grounded by little arms extending from the metal trackpad hinges.