Pull the bezel off, then 4 screws, pull the radio out unplug it, usually 3 plugs and the antenna cable, check current draw again, if still there then check the lighter socket and wire next since it's now easy to inspect next to the radio location.
Also it's much easier to see if an external alarm or other additional gauge etc. wiring has been added to these always powered circuits without the radio in place.
Should take an hour tops, maybe an extra half hour if all the bezel clips disappear into the dash...