The vests I made using the car seat heaters will cook you,
to the point where it would burn your skin if you ever had it next to your body. Maybe your wires are too far away, imbedded in the mesh?
I don't know if you can get them there, but those hand warmers
that you just rip open and produce heat work great!
I have to agree with other people. Overall, if you can keep your
core temperature up, your extremities should stay warm.
Your profile says you're in NY state, can't you get a heated car seat warmer there, and try that? Try Walmart, or if you're across the border ,Canadian Tire.
I paid 9 bucks for mine, and on the original one, just cut a hole in the crease to fit over my head, and wore it like that.
When you have it running, it should feel like a warm sunny day,
even at sub-zero temperatures.
Great minds think alike;small minds seldom differ...