how do you get to the power condiut to open the door to the cave?
Quarentine Cave... That's where you fight Thardus, right? I believe above the door is a stone arch, that you must shoot with a Super Missile in order to destroy. Once destroyed switch to the Thermal Visor and shoot the Wave Beam at the conduit, which was blocked by the arch. This will activate the door.
Of course, I haven't played MP for quite a while, so I may be wrong. Try scanning the area near the door.
Also, please refrain from making two of the same topic. Some people might try to ban you for that.