Yeah man, you just spin the little wheel towards "hot" right next to the defrost switch....
Are you kidding OP? Have you ever seen ANY car manufacturer that makes an adjustable defroster for the rear? These rear defrost threads are KILLING me. For the love of whoever you pray/don't pray to, READ THE DA*N MANUAL. If you think it defrosts too slowly, please just sell your 4R to someone who will read the manual, so these re****ed threads will cease.