There are 2 ways to handle the hardware installation of the GPS. Locate the GPS inside where is can get a good signal, or if that is not possible mount it outside. 

1) Do not forget to take into consideration ease of accessibility, it will be necessary to physically turn the unit on after a power failure or it's power supply was shut off.
2) Temperature and humidity limits. The attic is a poor choice as well as outside in a sunny location.
3) Cable limits. How many feet is RS-232 good for?

My Install. I need to put my GPS outside in order to get a good signal.

What I needed:
1- 1.5" plastic weather proof electerical junction box.
1- Stick of 1.5" PVC tube 10 feet long.
1- 1.5" PVC end cap.
1- Can of PVC primer.
1- Can of PVC glue.
1- 500' spool of 18ga 4 conductor wire. (I'll use the extra 450' for something else)
1- Tube of silicone sealer. 
2- 1.5" 'U' bolts.

A) After finding a  nice location on the north side of my house (free of direct sunlight) I proceded to drill a 1.5" hole through the wall into my basement
B) I installed my junction box with the lid facing up and horizonal,  it looks a little funny sticking out of the house but the weeds will soon hide it.
C) The junction box has 2 holes on it, the lower hole needs to be plugged, so I just cut off a 2" section of PVC tube and after priming it glued it into the box and glued the end cap on to that.
D) Secure the tube in the house, run the wires and seal up the hole.