By default, if you add another source as you have done, the packages will be used in preference to those from the wheezy repo, because the version numbers are greater. So there is no need to use the -t
option.
If you want to prevent this and only use specific packages from the wheezy-backports repo, you'd have to add a package pin. Run man apt_preferences
for more info, but see below for an example that would do this in the case you have described. Place it in a file called /etc/apt/preferences.d/wheezy-backports.pref
.
Package: *
Pin: release a=wheezy-backports
Pin-Priority: 200
Package: linux-image-amd64
Pin: release a=wheezy-backports
Pin-Priority: 600
The above will set the priority of all packages in this repo to 200, which is lower than the default of 500. The man page says that APT will "Install the highest priority version.", which means that the highest version only gets installed if the priorities are equal. But the example also sets the priority of the kernel package to higher than normal, so the one from wheezy-backports gets chosen over anything else. (Make sure you also add dependencies to the Package:
line, otherwise the correct versions will not installed.)
Little known fact: apt-cache policy linux-image-amd64
will show you which versions are available and which will be installed by default.