You can take a dremel and cutoff wheel, first cut the top of the acorn nut off, just cut the dome off. Then you can cut a slot across the top of both the bolt and the nut, just like if you were going to split it in half, but not too deep, you are just making a screwdriver slot. Those sparks from the dremel will damage your paint and chrome so cover it up! Spray some penetrating oil in there and let soak. Get the proper sized screwdriver in there to hold the bolt, but not interfere with the nut, and MAYBE you'll be able to hold the bolt and turn the nut, and back it off that way. It shouldn't be tight, but it may be rusted or galled. If that doesn't work you haven't lost anything, but then you'll have to get a nut splitter (google it) and bust the nut off. You could go ahead and keep deepening the slot with the dremel until the nut falls off, but be very careful to do it without cutting into your dash. You can get a new carriage bolt at any hardware store, put anti seize on the end of it and don't overtighten, you won't have any more trouble with it.

You will definitely want to replace that bolt even if you use the nut splitter, the bolt threads being anti-siezed and in very good shape is the key to being able to remove the nut in the future, you'll see when you get it off. Also, on second thought, a hacksaw would work, probably even better, for cutting that screwdriver slot...

Nah, just lift the dash up, it's right there. Shouldn't even have to unplug anything if you don't want to.