2005 kia amanti heater blower will not come on when below 40 degree's but works good above 40

Don't know what model you have. 03-05 Amanti has fuse AV audio. fuse box diagram below.
https://www.autogenius.info/kia-amanti-2003-2005-fuse-box-diagram/
03-06 Sorento
https://www.autogenius.info/kia-sorento-2003-2006-fuse-box-diagram/
03-05 rio
https://www.autogenius.info/kia-rio-2003-2005-fuse-box-diagram/

"Both" what heat up? Both heated seats? If so, the heated seats are heated using electrical current. The cabin heat from the vents is heated from engine coolant. The car will need to be running several minutes before heat will start blowing from the vents. If once the engine temperature gauge gets up to normal you still are not getting heat from the vents and are certain you have the climate control settings correct to be requesting heat, you probably either have a blend door issue or an issue with the valve that allows coolant to circulate through the heater core. I'm assuming that your blower motor is working and you can feel air flow, just the air flow is not warm. If you are not getting air flow, then the blower speed controller may have failed, the blower motor may have failed, or some other issue is preventing power from getting to the blower motor.

The blower resistor is bad. Find the blower(usually on the passenger side, under the dash) the resistor is close to it.

Kia Amanti Workshop Owners Manual

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Idont know that heater display has its own fuse. Most likely its ?the bulb. Hope that helps

it sounds like either the vent is inoperable on the drivers side heater unit or the pipes from the heater unit have become free and need replacing back on the unit

check all your wireing

well first check the relay, and make sure power is going to blower motor., if it is then the blower motor may be bad and drawing to much juice and shorting it out enough to blow the fuse or relay. good luck!!

if you went to Kia dealer last Feb and they did not solve problem and it is still a problem in Dec, then you are the problem. Make the dealer fix the darn thing!