The gate light comes on most of the time...i always check the back door and it's closed. My car battery even died on me once. I'm assuming it's a sensor on the back where the door shuts...how can this be changed without incurring too many costs of having to bring it in to a dealership?

https://www.youtube.com/watch?v=uiF-jRgB7bM

it could be a sensor out of adjustment or shorted. you will have to find which door is causing the problem and check out the sensor. you can also check for a code in the BCM as this is were it is monitored. it may tell you which door and possibly what set the light.

Probably a lift gate latch, the switch that turns the light on in the dash is part of the latch, also check the wiring inside the rubber tube at the top of the lift gate door when it's open, could be a broken wire in there

I assume the ajar light comes on if you open the rear gate? Try to duplicate the problem with the car running and parked, and you pulling on the gate. Sounds like the door or close switch needs adjustment.

Open gate and find where the wiring enters the gate (bottom of door) pull back the rubber insulator and check for broken wires or wires touching each other.

SOUND LIKE FAULTY LIFT GATE SWITCH.

There was a safety bulletin by Toyota regarding the lift gate of the 2007 Sienna. I had to jump into the cargo area once to avoid being crushed by the lift gate coming down unwantedly.
Contact your dealership..they replaced the lift gate hydralics (under warranty at the time) and the problem was solved.

IMPREZA: 2005, 2006, 2007 and
FORESTER: 2005, 2006, 2007, 2008
Check for Immobilizer key on Forester XT models only.
information is always subject to change and correction
new remote for 2005 Imprezas.
This is not the same remote as the 05 Legacy/Outback remote which also has truck/gate release on the front and a red panic button on the back.
Lock: press Lock button for 2 seconds or so to lock car, and arm optional security system. The car with chirp once and turn signals flash for locked, 5 short chirps and signal flashes if any door is not fully closed. The dome light goes on for 15 seconds and ignition key ring (if present) will light for 30 seconds when unlocked.
Unlock: press unlock one time for driver's door, press again within 5 seconds to unlock all doors inkling rear gate (wagon). The system does not open the trunk.
Panic mode: press and hold lock button for approx. 3-4 seconds to activate Panic mode. I panic model the horn honks and turn signals flash until either the lock or unlock button is pushed again.
Finder mode: Press lock button 3 times within 5 seconds to activate Vehicle Finder mode. The horn will honk once and the turn signals will flash 3 times
Distance: works within approx. 30 feet depending on battery strength, nearby concrete walls and etc.
Audible Chirp On-Off: the system will chirp when locked or unlocked. To turn OFF the chirps, unlock the door and then push both lock and unlock buttons at the exact same time for 2+ seconds. There will be 2 chirps indicating the the audible is off. To turn the chirp back on, repeat the process and the system will chirp one time when it is back on.
Battery: as the battery ages it will only work as you get closer to the car. When a battery is replaced, the transmitter must be re-synchronized with the Keyless control unit. To synchronize, press the lock or unlock button 6 times.
Programming: 4 codes may be programmed.
Sit in driver's seat and unlock all doors. Open and close the driver's door
Turn the ignition key from LOCK to ON position 10 times within 15 seconds. Be sure to stop at both lock and on. The horn will HONK once to indicate transmitter programming mode is activated. Leave key in LOCK position and don't remove it from ignition.
You now have 45 seconds to complete the programming as below.
Open and close driver's door again.
Push either lock or unlock button on the remote you want to program. The horn will honk 2 times when successful.
To continue and program other remotes at the same time: open and close the driver's door and push lock or unlock on the remote. The driver's door must be opened and closed between each remote you want to program.
Done programming? remove the key from the ignition and the horn will honk 3 times.
DELETE old codes? Reprogram 4 times, it will overwrite old codes.
Alarm Security System: optional on all models. When locked there is one chirp (can be turned off, see above), the turn signals will flash and the red 'security' light will start to flash once every 2 seconds except for STi which flashes twice every 2 seconds.
There will be 5 chirps if a door, hood, and trunk or gate is not fully closed
You can also arm the system by using the power lock button on the driver or passenger door as you get out of the car and then closing the door behind you.
Disarm the system: push unlock for 2 seconds
Lost remote, alarm activated? When you can't use a remote to turn off the system: open the door with a key or try to start the car and the horn will start to honk. Turn the key on-off 3 times within 5 seconds and the alarm will go off and you can start the car. It's simple!
VALET mode: To set valet mode or just to disable the security alarm system: 1. unlock all doors 2. physically open the driver's door 3. hold the UNLOCK button on remote for 2+ seconds. The red 'security' light will then flash 2 times a second until the car is turned on.
System has been Tripped (except for STi): if you unlock the door with the remote and it chirps the signal light flash 4 times, the system has been triggered;
Identification mode: To find out what happened and why the system was triggered:
Open and drivers door, leave it open. Turn ignition key from LOCK to ON 2 times and then leave in LOCK position.
Tripped code: 1 security light flash: a door has been opened, 3 flashes when the impact sensor was triggered, 4 flashes when the ignition was messed with or battery disconnected

if you turn off the switch for the interior lights,an override,it will prevent the battery from going dead until you can get the door alignment or switch repaired.the interior lights,there is more than 5 will kill the battery for sure.

if you can get the car started again and with someone helping you have them take the positive side of the battery cable off while it is running and if it dies then it is the alternator also check your drive belt and when was the last time it was changed because it could be slipping