Try checking out if the heater fan relay and/or the fan switch is sticking, on my '84 century it has a 3 position "switch".. on/off, low and high speeds...it would get dirty contacts same for the relay
You probably will need to replace your blower control module ,it is under your hood at the center area of the firewall ..three 7mm screws hold it in place ( you can unplug it to turn the blower motor off until you get the $ for the part ) HTH
John