For the show's first two seasons, an upbeat instrumental piece written by Fred Werner was used as the show's opening theme. Beginning with the show's third season, this was replaced by a slowed-down vocal theme titled "Eight Is Enough," which was sung by series co-star Grant Goodeve. The song had music by Lee Holdridge and lyrics by Molly-Ann Leikin, and was first heard in a longer arrangement on the last episode of the second season titled "Who's on First?", which was also performed by Goodeve.

Answer this question based on the passage: who sang the eight is enough theme song? Let's have some stream of consciousness first.
This is the relevant information: Beginning with the show's third season, this was replaced by a slowed-down vocal theme titled "Eight Is Enough," which was sung by series co-star Grant Goodeve. So, the answer is Grant Goodeve.