Search the barracks for seeker piece is a mission in The Lost Frontier. After the search for light eco and powering up the eco seeker it started pointing towards missing components, one of which was the old Aeropan barracks. Phoenix at first suggested the other direction, Far Drop, though Jak contested and suggested the rig, as it was closer. After a brief heated exchange, Phoenix deferred to Keira, who deferred to Jak.
Upon landing at the barracks, proceed through the surrounding area until you run into another Precursor idol, which grants you the eco shield power, which creates an impenetrable sphere around you (by simultaneously pressing and for PlayStation 2 or for PlayStation Portable). After reaching a locked gate and a cutscene, you will have access to the facility. Proceed to remove a nearby fuse box to open a door to a pool of dark eco, then activate your new shield power and use it walk across it. You will want to activate the eco shield as much as possible during combat as well, as you are invincible as long as it lasts, with the only downside being the lack of control due to the movements of the shield.
As you progress through the area you will encounter a dark eco mutated version of the armadillo as well as Aeropan commandos. The armadillos have a new ability to shoot their spikes at you, though they go down fairly quickly, the commandos are sturdier and can shoot dark eco bullets or large bombs that explode in small shockwaves. The lobber is excellent against the commandos, especially when they are using cover, which can be destroyed by your weapons.
The first part of the barracks will see you use eco teleport to switch with statues floating on top of small geysers of dark eco, while using eco shield to avoid instantly dying after each teleport. Eventually, you will run into a large room filled with impenetrable laser barriers, as well as three buttons and three statues. Use eco teleport to switch with each statue, while standing on a button, until all buttons are pressed down, at which point you can walk through the next door. Shortly after you must use the eco amplifier power to destroy a cracked dark eco canister, causing it to fill the nearby pool with dark eco, after which you can use your shield power to walk across.
At the end of the area, you will enter another large room filled with commandos, armadillos and one ethereal sub-commander. The latter will do nothing while all the other enemies charge you nearly all at once, so make sure to use eco shield and eco reflexes to gain an edge while you defeat them all. Once done, the sub-commander will teleport down and resurrect some of its fallen comrades, so proceed to throw everything you have left at it. It can use either a ground smash, which creates a shockwave or throw a series of dark eco blasts at you, so stay at a distance if you are not using your shield power. It has more hit points than the commandos, but will go down in due time, after you which you can head to the next room and start a cutscene, ending the mission.