either go through a grommet by peircing it or drill a hole. if you go through the grommet a trick to do it is to pierce it then take something long ( long zip tie or a coat hanger ) and then feed it through there until you can see it in the compartment. then tape your wires or hoses to the other end inside the passenger compartment and then pull through engine compartment.
best thing to do is to just gauge the area that you are thinking about drilling through. look at an easy place inside the passengers compartment and then take a visual cue of what is around it, IE... steering rack, brake pedal, wires, etc.., then take a look at about the same general location and see if anything is in the way. when you do decide to drill take a self tap (1") and drill through and then take the screw out, take a poker and then poke through into the engine compartment to make sure that there is nothing on the other side. if you feel no resistance then drill the hole bigger. also, if you dont have a poker or awl, you can take a light and drop it in the general area of where you are drilling and look at where you drilled from the passenger compartment. if you see light then you are good to go, if you dont see light go elsewhere.