it should be easy, mine was, its a cab though, it you look on the bulk head kind of to the left of the battery there should be a plastic bung thing, about the size of a mug round, pop it out (you may have to push it instead, it wont go anywhere dont worry!) and punch a hole straight through it, push through the firewall fabric stuff behind the plastic bung and just pop the wire through from the engine bay side and it will appear in the passenger footwell. be careful using the existing wires to 'piggyback' off as the wire to that point may not be high enough rated to cope with the amp, depends what amp. one of the easiest cars i have done this in! the hard bit was attaching it onto the existing battery terminal, with all the other wires.
hope the pictures help!
