I think I've found your film. It may be Robo Warriors, a 1996 film shot in the Philippines (with some lush jungle locations). Here's a pretty detailed summary. Some of the details match up - they are fighting for independence (from aliens), and the robot has been hidden since before the aliens conquered Earth. However, the conflict is actually settled with an arena battle, as the robot and its pilot become Earth's champion, fighting one of the aliens' robots (which look like dinosaur skeletons stuffed with guns) mano a mano to decide the war.
By the way, Robo Warriors was conceptualized by the writer-director of (among other cult genre films) the 1989 film Robot Jox, which is much more well-known and well-regarded. If you're looking for something like this but a bit more entertaining, Robot Jox or two related early-90s films, Crash And Burn and Robot Wars, may be good choices. Those films feature stop-motion animated robots, while Robo Warriors' robots are men in not-too-convincing robot suits, and they have generally higher production quality than Robo Warriors.
I've never seen this film, and there doesn't seem to be an English-language DVD, but there's a full copy of it on Youtube (unfortunately dubbed in German) and I've reviewed it to check how closely the sequence in question matches your memories. There are some differences, but it's pretty close.
The folks who find the robot aren't a man and a woman, but rather a man (the robot's original pilot) and a young boy (who's trying to get the man to fight the aliens with his robot).
When they find the robot, it's buried in the jungle, and the control room is turned sideways.
They don't exactly get into the ductwork, but the boy has to do some crawling around to access parts of the robot's systems.
The robot's spacious command center is kind of like a bridge, with a big windowed dome and many control panels around the pilot's seat.
Finally, the robot rises up out of the jungle to fight.