If you're going to put a HD in your trunk, be sure that you won't be putting the drive in an enviornment it's not rated for. The specs below seem fairly typical of the few drives (non-laptop) I looked at:
Shock
Operating Shock (Read) 65G, 2ms
Non-operating Shock 300G, 2 ms
Temperature (English)
Operating (English) 41° F to 131° F
Non-operating (English) -40° F to 149° F
Temperature (Metric)
Operating (Metric) 5° C to 55° C
Non-operating (Metric) -40° C to 65° C
Humidity
Operating 5-85% RH non-condensing
Non-operating 5-95% RH non-condensing
Altitude (English)
Operating (English) -1,000 feet to 10,000 feet
Non-operating (English) -1,000 feet to 40,000 feet
Altitude (Metric)
Operating (Metric) -305M to 3,050M
Non-operating (Metric) -305M to 12,200M
Vibration
Operating
Linear 20-300 Hz, 0.25G (0 to peak)
Random 10-300 Hz, 0.004 g2/Hz
Non-operating
Low Frequency 5-20 Hz, 0.195 inches (double amplitude)
High Frequency 20-500 Hz, 4.0G (0 to peak)
To combat temperature and humidity problems, you could build a sealed insulated enclosure for it (which is where the drive supports would anchor). The insulation should help protect it from cold days (as you can see, hot is no problem) and being sealed should help protect it from Florida's humidity
You could even throw a dessicant such as Silica Gel into it to help keep humidity low.
JigPu