I'm pretty sure they will endorse Biden for a couple of reasons.
One: Trump made a lot of promises to the UAW in 2016. But during his presidency he forgot all about them and made no effort at fulfilling those promises.
Two: Trump held his Detroit UAW rally at a non union Trim plant - which the UAW probably didn't like. And the speech he gave supported his agenda not the auto workers long term goals.
I included a link to what the VP of the UAW had to say about Trump. Cheers!
https://www.freep.com/story/money/cars/general-motors/2023/09/27/mike-booth-gm-trump-uaw-strike/70981329007/