I think the mystery is resolved. Just have a peek at the thread I started with your query. The boys have come thru! This is something you can fix easily. You just need to find a used one from a wrecker.
I hope you get this. I looked at the service manual and looked over the part schematics and I have no clue what the dealer is talking about.
However I assume you have a version with automatic climate control. If so the part is to be found at the base of your blower motor and accessible from underneath the passenger side of the dash. It is an amplifier and on non climate control versions the part is a resistor and only controls fan speed.
Have a look at this thread it should be able to help you out.
Xtrail 06 blower motor noise - Page 2 - Nissanhelp.com Forums
The part should run you about 100. Whomever is advising you is full of crap and either call them on it or find another dealer to help you.
Call the part dept directly dont go through some bozo service adviser.
And please start a public thread on this topic.
Mine is in the sweet spot these days. Everything is working perfectly and alignment is spot on.
When I get a chance later I will look for the part you need. I am sure something can be figured it.
In the meanwhile maybe you can identify it with this link. http://nissan4u.com/parts/x_trail/
Actually we should just take your query and make a new thread in the forum. Someone else may be able to help or be able to be helped by the info we come up with