If your real lucky it will be an easy fix.  Get a big can of WD-40 and
affix the red tube to the can.  Shoot a lot of it into the rear lock
and the drivers lock.  You should be able to see into the back of the
lock from the inside of the trunk, clean the back side of the lock
also.  Dirty contacts sometimes cause this problem, if lucky that will
be the case.  If not, more expensive remedies await your pocket book.
Check fuses also.  WD-40 is a cleaner and displaces moisture.