1990 Cadillac Deville fan won't quit

Common failure of A/C power module, in the HVAC case at about the middle of the firewall. Has two electrical connectors, one two pin, and one four pin. This refers to how many cavities, not wires. I believe there is one in the two pin(fat red), and three in the four pin. You could disconnect that and the blower will probably stop. It is shorted internally. Another thing to check that can cause failure of this module, is blower motor draw. If it draws over 25 amps, it may need replaced.