I think you wont find any definite proof what the mass of people thought back then about this, but ofcourse it was kept in secret. Also only about 3000 Germans (most of the working force and anti-partisan movements were German collaborators) were employed in the "death industry" (in contrat 18 million served in the army) so most did not have a personal connection to the death industry. Most Germans denied after the war they knew anything about it, but many time witness interviews, especially by people who served in the army said they had a kind of notion based on rumors that Jews are getting killed in masses. In Traudl Junge´s book about her time as a secretary she wrote that one wife of a high ranking German offical confronted Hitler directly with what will supposedly happen with the Jews she saw which were transported on a train away from the Netherlands (and he remained silent).

How aware were the Nazi voters and supporters of just how brutal their government was

You have to be aware that after the invasion of the USSR, Germany was in deep shit trouble. Hitler already saw latest in December 1941 that Germany will loose the war and as a revenge he sought to root out the people who were allegedly responsible for the demise of Germany (the Jews). Latest from 1943 almost every German was aware that if the war was lost, Germany might cease to exist as a state and that the Germans might if not exterminated, but then severly chocked by the allies for an indefinite periode of time. So even if they knew they did not care anymore what happend to the Jews, because they were busy securing their own survival.

Edit: Some spelling mistakes corrected.