Engine compartment, does this mean the GPS unit will be mounted on the hood of your car ?
GPS signlas are like light from a flashlight (you probably already know this) so the GPS unit must be outside on the car. When you say engine compartment it means to me that the GPS unit is in fact somewhere on the front of the car. BUT because the roof of your car is metal and higher then the hood you are reducing the angle at which it can recieve signals.
You should try to put the reciever on the roof (or highest point of the car) This will get rid of the problem of trying to go through your firewall. instead you can go through the door Jam or something like that.