The 1964 Riviera has a 4-speed fan (high, medium, low, and low-low). The heater/defroster only uses the 3 lowest speeds. That is, the high setting on the heater fan switch is the same fan speed as the medium setting on the A/C fan switch. 

I figure that if there's one time I want full-on air, it's when I want to defrost my windshield. Therefore, I rewired the connector to the heater blower switch. You have to disassemble the blower switch connector to do this, but you can do all of this just by pulling the A/C controls out of the console. Insert a small screwdriver in the front of the connector to depress the locking tang, then pull the wire out the back. I tapped a lead off of the high-speed connector (gray wire) from the A/C blower switch and put that in the heater high position. I also moved the low wire (light blue) to the low-low position, and the medium wire (orange) to the low position. I disconnected and taped up the now unused low-low wire (yellow). I essentially moved every switch position up one blower speed. See Fig. 11-57 in the shop manual.