I get the Old Trafford Stadium to work on a Friendly Match but it won't play the custom one in the season. It just plays the default one. Are they not supposed to work during the stadium?

Is that the Old Trafford Century Stadium?

Anyways, you should have copy and paste the stadium files for Old Trafford, 8 of them, 04xx_x.big, into the enviro folder. Then go into FIFA, choose Game Option, and select stadium1 as the default stadium. Then start the season with Man Utd. The stadium now should be Old Trafford.

Alternately, use FIFA FX, much easier.:)

And better.....:)