1865. Union medical wagon drivers were a separate set of soldiers from the ambulance drivers. The ambulance drivers actually went onto the field, both during and after battle, and got injured soldiers out of harms way and back to a set destination. Basic first-aid was applied to stave off death, but fixing the injuries themselves was not their job.
The medical wagons did not go into battle, but instead were basically mobile surgical suites that could go between camps quickly, had all the supplies needed for amputations and excisions (the main surgical procedures), and had much more experienced medical personnel.
By the way - the popular perception that US Civil War surgeries were done without anesthesia, by giving the soldier a shot (or five) of liquor and a bullet to bite down on, that’s almost completely untrue. The vast majority of surgeries in the Union (and a majority in the Confederacy, just not quite as high a percentage), were performed under ether or chloroform.
You can’t really say that the whole liquor + bite down on something method was never used, especially seeing as there were some really desperate situations out there, but it’s not at all what happened in camps, during the average surgery, with a stocked medical wagon and competent surgeon.