Most car seats unbolt from the floor and the back cushion lifts to remove it. For them you need a wrench (yup, the wrench locked in the trunk).
Some seats just clip in place and don't even need tools to get out. If you really had to I suppose you could cut the cushions out of your way with a knife and wire cutters.
Whether you can get through whatever is behind it is another question to answer. Most of the cars I have had would at least let you reach an arm in.
The car I have now (Pontiac GranPrix) has a middle seat in the back that folds down and makes an opening into the trunk. No tools needed. It was an option to let people put longer items inside the trunk.