Here is a good indicator of how long your battery will last in the trunk. Take a multimeter and set it to DC. Test the Alternator. Test the battery.
Lets say the alternator reads 14.5
The battery reads 14.5 No voltage drop between the two points.
If there is voltage drop then there is a problem and your battery and alternator could be at risk. Your alternator because it has to work harder because the voltage regulator is commanding it to work harder.
If there is a voltage drop then there is more than likely corrosion.

The upside is no heat in the summertime.
Battery in the trunk makes some or no difference and could last longer because there is less heat to deal with because no engine compartment.
If you live in Minnissota the electricity has to travel alonger distance to reach the starter at 20 below zero.


Nine years is a pretty good life for a battery. What kills off batteries? Not using them. An example is storing the car for long periods of time without trickle charging, (sulfation). A two amp trickle charger will maintain the battery just fine for storage. Corrosion at the terminals, Heavy wires, Alternator.
If your battery happens to die because you were shining bugs with the engine off. Trickle charge the battery right back up when you get back home even if you jump started to get home.
There is a myth that once the battery is fully discharged the life of the battery is shortened. I disagree to a large extent if you know what to do.
Socking a battery with frequent jump starts will shorten the life. Shoving heavy amperage into lead plates causes small particles to flake off.
Good maintainence involves baking soda, water and a toothbrush. Clean them off until there is no reaction left.
Because there is a battery in the trunk place a copper penny between the terminals. Replace the penny as often as needed. The copper penny will take the beating before your terminals will.
Inspect your heavy wiring where they meet at the terminals. Don't let the corrosion start crawling along and inside the insulation.
Napa has battery cleaner in the can and corrosion guard in a can. Two wonderful preventers.
On another note. The older your battery gets the quicker the terminals will corrode, much quicker. It is a fair indicator that you are on borrowed time.