I've been trying to figure out the answer to this question for more than a year already! In fact, I even have the part (from e-bay), but it's not obvious to me at all how to install this part (with the second hard drive fan). Originally, I expected this to be similar to an extra fan on DELL Precision T5400, however, on T7400 it seems to be a completely different story. In other words, I have this question too!

p.s. part number:   XK827

I know this is an old post, but I thought I would post the recently discovered answer.  The second hard drive fan for the Dell Precision T7400 is mounted on the external, rear, of the case.  The fan housing has two metal tabs that slide into slots on the exhaust grate.  The fan housing is also held in place by two screws.  The fan cord goes through a hole in the case and plugs into the motherboard.    

@cmcfall

Thanks for the follow up.  I imagine this didn't get much discussion because it is posted under the FluidFS Product forum.  Only a specific search would return this thread.  Still, it is here, and will come up in a search.