Automotive electric motors

Tool review: Jump-N-Carry jump starter -- Portable model ...

www.autoserviceprofessional.com/.../tool-review-jump-n-carry-jump-sta...

Jun 6, 2014 - The Jump-N-Carry's convenient charge indicator reveals the unit's stored voltage. ... By plugging the male/male adapter cable into the Jump-N-Carry ... This 12-volt DC port also can be used to power 12V accessories (up to 12 ...

Using The Unit As A 12 Volt Dc Portable Power Supply ...

quick check is to remove a battery cable while the motor's running. If the motor stays running when you pull off the battery cable, the charging system is functioning properly, if it dies then there is a problem with the charging system. Don't leave it off long, as this test is stressful to the circuit. You can test the voltage regulator by putting a volt meter across the battery posts to verify voltage. A fully charged 12 Volt battery should show slightly above 12 volts DC. With the engine running and the volt meter hooked to the battery, the charging system should be showing around 13.8 volts DC.

What I would do is pull the fan blower motor speed switch, plug the wires back into it while you're holding it in your hand, run the car, and check with a multitester and see if you are getting voltage at the switch. Should be something like:
High=12-14 volts
MHi=8-10 volts
MLo=4-6 volts
Low=2-4 volts
OFF=no voltage

If you get voltage but the fan still doesn't work, then it's either a bad ground, or the motor is bad. If you can access the blower motor wire connections(sometimes its easier to try this with it removed from the vehicle), then you could try jumping the blower motor at the wiring connection with a known good 12 volt source, and if you STILL can't get the motor to spin then it's seized up or it's just a bad motor. If you have the motor removed, try spinning the squirrel cage by hand. Is it stiff? Seized? Probably need a new one if it doesn't spin freely by hand.

Hope this helps!

you need a voltage doubler circuit which feeds from your 6V DC system and converts it to AC and steps it up and converts it to 12V DC. This device might be difficult to find. OR use a 6Vdc power inverter and just use a 120volt AC fan if much easier to source out in your area.
You might want to try a 12VDC high speed fan and see if the 50% reduced speed is already sufficient for you.