Welcome to the CHICKEN Scheme pasting service

(lambda (thingy lst)
(let loop ((lst lst)
           (max 0)
           (idx 0)
           (max-idx #f))
  (if (pair? lst)
    (if (> (car lst) max)
        (loop (cdr lst) (car lst) (add1 idx) idx)
        (loop (cdr lst) max       (add1 idx) max-idx)
    max-idx)))