do you know how the original garage door was closed off? I would guess the original framing is still there, and the wall was created by adding studs in the opening.
Tear down the drywall from the inside to expose framing. get rid of no longer needed framing members. cut siding from the inside so its flush with framing. Or drill pilot holes from the inside corners to mark corners and cut from outside. Install door & trim. U don't need to remove all siding.