I would say the blower motor's armature is doing a slow burn. The resistance of the windings is dropping causing more current to flow through the dropping resistor, causing more heat at the resistor, causing the resistor to fail.
The relay would not cause this problem as the contacts could go higher in resistance (burnt contacts) BUT they would not go lower in resistance. Higher resistance would lower the current flowing through the resistor and actually allow it to be cooler.
I just checked ebay and a NEW blower motor can be bought for $35 with shipping..
04 05 06 07 08 09 KIA SPECTRA BLOWER MOTOR 2.0L 4 CYL | eBay