I have created two military-related custom careers and would like to use a custom vehicle (i.e., a Hummer) as the carpool vehicle. Does anyone know where I can find a tutorial or other information which explains how to replace the standard carpool vehicles with a custom one?

from what i can see here => Tutorial: Complete Guide to Career Modding for Beginners! is:
when you made a custom career (the military ones in your case) you choose the cars used from a list. so if you want to costumize the car - i say you need to costumize the list beforehand. to get the car in the list for as e.p. CarUsed1

but how to do that? i dont even know where such lists are stored in the code... did you check if this tutorial by nonamena how to make an XML tuning would help? it tells you how to dig in the game data with s3pe. the list for used career cars must be somewhere there

Well, I believe you need to script in C#, to create a carpool vehicle that is derived off a current one so that it doesn't replace an exisiting carpool item, and then use a military car to replace the mesh of your custom carpool vehicle. But I don't know if it even actually works, and in fact, there's no way to find a tutorial to everything - as I always say, if there's a tutorial, then it shall already be done, and modding is about venturing into something known of the game (that is, with some basis or knowledge from tutorials - eg there's no tutorial for a script I want to write, there are only chunks of useful codes here and there to be found, that require to be assembled together through trial and error). Good luck!