If you're having trouble with the cables or the mechanism from inside the passenger compartment, there are two cables that run into the trunk - one releases the top latch, and the other releases the boot cover.
On some cars, they're hidden behind the carpet. They should be easy to find - If you're looking directly into the trunk, the upper panel behind the top boot is where the latch is. The hole is just left of center, and the cables should extend leftward from there.