Board Thread:Morrowind/@comment-120.60.169.122-20130215090853/@comment-5917342-20130218175418

Normally when he used the cube to activate a door, then he would ask you to get the cube and said it would be needed. So the cube belonged to you. Then you have all the cubes and before going back to activate the boiler, he said he would get the cube and meet you at the boiler. But I didn't let him retrieve the cube. I got it myself. So I had all the cubes and could activate the boiler by myself. I think that's one solution.

What's more, the cube's ID is "xx0173bb". You got the cube and you'll complete the quest.