You start with the battery cables. Disconnect both (yes, I know everything else works, but that'* typical), and after you disconnect them, pull the boots off. Many many cases of this come up each month (a dozen or so) and the corrosion prevents enough current for the starter, but provides enough for the other accessories.
You SHOULD disconnect all cables at each end (positive and negative alike, where each end terminates), break them loose, clean them up, and apply dielectric grease to every point before reassembly.
I've been bitten by the same bug myself twice, and watch them regularly on this forum. That is your most likely culprit.
Tell us what you find, and we'll take it from there.