Where does the extra white wire go from the MC42 electric motor

I would contact the manufacturer for one, probably under spare parts or consumables.

Impossible to source without a maker. The search query for this is simply

make model replacement remote

or go to the maker website.

Why don't you ask the thieves to return the remote?

https://www.google.com/search?q=%E2%80%8Bmake+adjustable+bed+replacement+remote

..

What about it.

We do not have parts here.

you should have a wiring diagram on the top of the motor to tell you what each wire does. compare both motors and see where the wiring goes.

I have the information on the wireless remote if you need it.

Bad motor or you need to remove the shipping bracket that immobilizes the motor during shipping.
Call the Fan manufacturer if your past the 30 day department return window, but still within the one year manufacturer warranty. If you paid by credit card-they extend the warranty an extra year for you. Also, check your wiring to make sure you connected black/blue wires together to the black wire coming from the ceiling and the white wire to the white wire coming from you ceiling.

Go out and spend the money for a Hunter Fan. They have good ratings and customer satisfaction.

Remove the dump bed, then the seats, on the frame of the gator that was under the bed will lift up. there you will see both motors and wires