what's the best way to run a heater motor directly to the battery and an inline switch? how many volts is the motor?

 

12v...using sufficient guage wire, switch and proper circuit protection.
what is wrong with the factory wiring?

 

12 volts to the blower motor will give you high speed only.
X2 on what's wrong with the factory setup?

 

In 2002 I wired such a bypass under the hood of our 2001 Mercury GM using a HD 30 amp all brass "boat switch", a few feet of 12 gauge wire, and 30 amp fuse from battery + terminal to blower bypassing the in car switch, resistor, etc as it was summer, we were on the road, we had a puppy with us. Like said, run the blower on high until we got home and I could get it to the dealer for warranty repair. I used a boat switch as I just zip tied that switch under the right front fender. No more issues, car has not yet tripped 100K, looks new still.

Not long ago, I used the same wire, switch, fuse to temporary operate blower for heat in our '95 T-bird, but just used it once to get to a hospital appt over the mountain. This time, I laid the switch in the glove box. The T-bird is a good running, comfortable car, only has 116K miles, but it needs some paint on it's hood, roof, & trunk. It's also taken some minor hits from others in the parking garage at UVA so I try to only take it over there.

Like with the Mercury, we just used the windows to modulate in car temperatures when switched on.

With the T-bird, it needed an electronic pulse wave control module, it's fixed now. I think with the Mercury was a bad resister board though. My '77 has a resister-controlled blower speed too.

 

Once the blower starts getting old and harder to turn, they tend to start melting connectors and switches. Overheat terminals. A simple connection to battery is a stop gap measure. But I understand the need sometimes.

 

When mine started to die, it stopped working in the winter or cold weather, but the next summer I bumped the switch and it came on. Then, the next winter it was dead again. I confirmed the bad motor by straight-wiring it in the cold. I guess the parts of the motor all contracted enough in the cold, that the brushes were no longer making contact with the armature or whatever.