/lib/modules/3.16.0-6-amd64/source is in linux-headers-3.16.0-6-amd64 3.16.56-1+deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-3.16.0-6-common
/usr/src/linux-headers-3.16.0-6-common