Can be unlocked in "Glacial Grave" level, on the opposite branch of the river that leads to the gunboat hanger.
Can be unlocked at the beginning of the "The Smugglers Alliance" level. Help the civilians, even though the captain tells you not to. A speeder will come out of a homestead. Protect it and it will lead you to a farm with the upgrade.
In the second secret stage, turn right immediately after starting to find a brown striped building. Enter the hole in front of it and turn left immediately before the path turns to the right. You will see it to your left.
Successfully complete the standard levels with all silver medals.
Successfully complete the standard levels with all gold medals.
Successfully complete the standard levels with all bronze medals.
Can be unlocked in the "Borvo the Hutt" level. Go northwest of the first battle to find a village among the trees and the upgrade.
Can be unlocked near the end of "Sancuatary" level. There is a small valley to the right of the main valley. Inside is a volcano and a strange "fairy" object. The upgrade is behind it.
You can play as the Sith Infiltrator once you access the third secret level, "The Dark Side". You can use the Sith Infiltrator in levels where you can use the Naboo Starfighter.