I had a 1961 Bug,..and I used to put a light bulb on under the hood, and near the 6 volt battery. Sometimes it didn't help, and I would have to roll it down the hill to start. I also had a 1976 camper mobile,..that one would only go up the hill to my house in reverse.
Once they got running they ran all day with no problem, but neither had working heat,...upstate NY,....very cold.