A/C blows dash fuse when on

I just bought a 1986 DVL/Flt Car Craft convertible, and do not know the car yet. It does have only 24,000 miles on it. Right off -- when the ac was turned on it blew the 20 amp fuse for the dash readouts--mileage, speed, etc. What may cause this. I also have noticed that the passenger visor has come loose at the connection where the wires are. I tried reconnecting the one loose wire to the terminal, and that did not work the light in the visor, but when I put it back in place, the wire touched the metal frame on the car and the light came on. Also, the front windows work--slow, but that is expected now, but the rear windows do not work at all---from the driver's panel, or the individual switches in the right and left rear. Could all this be connected in the electrical system somehow, or would they be different situations. I have not tried the top yet, as it says to lower all the windows first. Since I can't lower the rear windows, I have let the top pass for now. I hope it is something simple. After 5 years of trying to save a little I splurged and bought this convertible car, and that really pressed me, Costly repairs will have to wait a few months, but maybe this could be an easy or inexpensive fix. Thank You
David