Where the last step meets the garage, make sure you lay some drainage pipe (under the step.) I've got the feeling that by putting in the step, you might 'block' natural runoff from getting to the 'other side' of the steps. If this is the case, you need to picture what the natural flow of water will do once it encounters the step. If you don't catch it and get rid of it, it may end up in your garage.
Just a thought. My best to ya and hope this helps.
PS: Whenever I 'lock out' part of a lot from another part of a lot, I always put in a drain tile and 1" PVC pipes underground to 'bridge' the 2 sides. The drainage pipe is for possible water issues. The other 2 pipes are for low and high voltage wiring. Who knows, some day you may want a phone and cable TV and 'more power Scotty' in the garage, and need to get 'under' your new steps. It's cheap to do and you can cap-off the pipes. (Just don't seal/glue anything; just cap them ...)