mtcars[, c("cyl", "am", "gear")] <- lapply(mtcars[, c("cyl", "am", "gear")], as.factor)

p <- ggplot(mtcars, aes(mpg, wt, group = cyl)) +
  geom_line(aes(color=cyl)) +
  geom_point(aes(shape=cyl)) +
  facet_grid(gear ~ am) +
  theme_bw()
p


len <- length(levels(mtcars$gear)) *  length(levels(mtcars$am))
vars <- data.frame(expand.grid(levels(mtcars$gear), levels(mtcars$am)))
colnames(vars) <- c("gear", "am")
dat <- data.frame(x = rep(15, len), y = rep(5, len), vars, labs=LETTERS[1:len])
p + geom_text(aes(x, y, label=labs, group=NULL),data=dat)
dat