pics related to viewing post

Hi there, first thank you for your hard work. I need help, a client of mine needs Ad pics shown on the right sidebar, but the ads should be related to the category of post the viewer is watching. So, on post viewing cat. cars, only car ads on the sidebar, and so on. I am trying to put the ads in posts and give them a category, but i think this would lead to showing up also normal posts from that category on the sidebar. Any way to go around this? thanks for helping