The HST light is not used on our N American marketed BX's. Theres no sendor installed on the HST to control it. Kubota apparantly used the same wireing harness for all their 00 & 30 series BX's hense the reason why it's there but stuffed up under the dash.
So why is it on? More than likely the control wire thats bundled under the seat w/the Aux Lighting wire is grounded. Theres 2 wires that are taped together directly under the seat, one is used for the rear Aux lighting option (hot when the key is turned on) and the other is for the HST warning light that our BX's don't use. If that HST wire gets grounded it will lite the lamp under the dash. If that "hot" aux light wire gets grounded it will blow the 20amp fuse and you will no longer be able to shut your BX down via the key switch. I see you have another post where your BX wouldn't shut down until you replaced that fuse.
I highly recommend that you inspect under your seat, look for and locate those 2 wire ends and make sure they aren't laying in a pool of water and/or grounded in some way.
Do a google search on "BX manual fuel injector pump shutdown" and there should be some instructions and pics that'll show where the manual fuel shutdown lever is located on a BX.
Good luck
Dave