2005 gmc sierra...took blower motor out and hooked it to a 12volt battery...works fine. replaced resistor and checked all fuses but the blower still won't work...any thoughts?

Test with a voltmeter , metal tab's on top of fuse , should have battery voltage on both tab's . Don't go by look's good , test to know for sure . Look at a wiring diagram an test the whole electrical circuit to find the problem . Doesn't it work at any speed ? Does it have manual or automatic climate control ?
TechConnect Ep 37 How To Test Chevy Blower Motor Resistor DR775

Wiring problem of the 2005 GMC Sierra 1500 6 Failure Date: 07/18/2009

The connector that plugs into the blower motor under the glove box became too hot and melted the plastic resulting in the a/c and heat not to work (this includes the windshield defroster). I've been looking up problems and solutions and found that this is a recurring problem with GMC and chevy trucks yet there has not been any recall, in fact the dealerships have an extremely overpriced kit to fix their faulty wiring. About 66,000 mi.

In the beginning you should check for voltage and ground.
The 50amp fuse marked AC powers the blower motor and the motor is grounded.
The high speed relay bypasses the resistor. So you should have power coming from the fuse and thru the high relay. Then the resistor for the other speeds gets power from the heat and ac control module which also controls the blend doors.
You may need a shop manual or repair book to get the diagrams.

Check the fuse first thing. If the fuse it good then either the blower motor speed control switch, blower motor resistor pack, or the blower motor itself is bad. Normally the speed control switch rarely goes bad. If some of the speeds did not work prior to total failure then the resistor pack is bad. If that is not the case then apply 12 volts directly to the blower motor to see if it is good. If it runs suspect the resistor pack as the culprit.

did you check the fuses and did the motor come with a new heater resistor or is it separate

blower motor resistor is burnt out on those speeds

This would either be a resistor or the switch could also be faulty. Check lines to the switch. You can test switch output with an ohmmeter to see if there is resistance in each setting of the switch. Common GM problem

You can go to autozone.com and register for free repair manual if you desire.

bad actuator in dash very common on gm the dash may have to come out depending on which one is bad

Have you had the blower motor switch replaced? I had a Chevy Silverado and the low worked but no other settings. It is a pretty quick fix and reasonably cheap.

probably a bad resisiter in the plenum