Need help with RTCW!!!

[LukFilm]

I'm stuck in one level and don't know where to go. It's the level that is pictured on the box, the second picture in the first row. I went down to the sewers, but there doesn't seem any way to go on. Could anyone direct me where to go? Thanks.

 

[Platypus]

Get into the venilation shaft........
















































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[LukFilm]

Where is it?