5:32 Who is "another" that Jesus says bears witness of Him here?

Another that bears witness of Jesus are both the works of Jesus and God Himself.

The works Jesus did testified to His Deity and Messiahship (CP V31-36 with 20:30-31).

God also testified to whom Jesus was - first at His baptism, then at His transfiguration (CP Jn 5:37 with Mt 3:16-17 and 17:5).