Do the other tail lights work properly?  If not, check the fuse.

With a volt meter, check voltage at the brake light sockets, using the
socket's ground.  If there is voltage at the sockets when the brake pedal is
depressed, the bulbs are bad.  If there is no voltage, check voltage by
using body ground.  If there is voltage when using body ground, the light
assembly is not properly grounded.

Check where the wiring runs along the rear hinge to make sure it is not cut
or chafed.

If there is still no voltage at the brake light socket, check continuity at
the brake light switch.

Also, make sure you are using the correct replacement bulb number.  Some
have multi-filaments.