Do conquered locations still help the Resistance?
Conquered Locations text is still in effect after being conquered. Demolished Locations text is not in effect after being demolished.
The rules don't make any mention of conquered locations losing their rules text. The demolish markers replace the text of the locations they are in, so naturally the locations rules are no longer in effect. This in confirmed by the lead designer and president of Flying Frog Games, Jason C. Hill in this BBG thread

Location Text remains in effect, even after that Location is conquered. The only time a Location's text is canceled is if it has a Demolished marker on it (which has its own text that replaces the text of the Location).
Location text DOES apply as soon as it is placed, so yes, Aliens do have to roll to see if they stumble into the powerlines with the Electric Power Plant as soon as the Location is revealed in their space.