First, are you sure you need a converter? I looked into this a few years ago and at that time many cell phone chargers cut the voltage down to 6 before charging the phone. I had a friend with a 47 sedan that just used the stock cigarette lighter socket for charging.
If you do need to convert 6V negative to 12V positive you are probably purchasing a converter specifically for this. Recognize that is an 18V change so don't expect a lot of amps. Make sure the converter you purchase is rated for this, and comes with good wiring instructions.
I have a custom made converter that raises the voltage, switches the polarity, and "cleans up" the power in my car that also monitors voltage in and out, as well as amps used. With everything going, I haven't seen it over 1 amp on output. My generator works just fine, and with an Optima battery, load has never been a problem.
I would love to see the schematic, parts list, and pictures of your converter. I find several different plans when I do an internet search, but prefer a version from someone who can answer questions. I understand if you don't have the schematics and/or just prefer not to post something that could be misunderstood.
Inverters generally use the system voltage to generate a higher voltage. I used them a lot when performing helicopter maintenance on the road. They allowed the limited use of 110-volts from a 12-volt vehicle system. They are rated in the wattage that they can sustain so a person has to make sure that it doesn't get abused by trying to operate to much off of it. The higher the wattage, the more electrical power it takes to run them. If the car has a limited output generator then you can't use much wattage to run this kind of stuff.
Some are simple units that likely use an electronic half wave generation to step up voltage through an inductor coil like radio amplifiers use to increase DC power. Larger ones actually have a full sine wave generator in them for AC current. We also carried Honda motor generators to have more wattage capability during heavy maintenance.
I have the above noted Amazon 6 to 12 volt step up converter in my 54 Meteor. Have used it about four years, it works great! I hid it under the dash, and put inside my ashtray the 12v socket. This device here is hidden in my ashtray: https://www.amazon.ca/LENCENT-Bluetooth-Transmitter-Wireless-Hands-Free/dp/B0CQVN2M9D?sr=8-28
I had my AM radio converted to AM/FM so using the bluetooth adapter, I can listen to my own music from my phone, and also take phone calls handsfree! I call it my 6 volt bluetooth lol.
Boy, looking at the blown-up pictures than Glenn provided made me realize how filthy my car is. For the last couple of years, the arthritis in my shoulders an especially my hands have made it impossible for me to grip a sponge or towel, or a rag and use it effectively. I have had enough, though, and the car is going to the detail shop next Saturday.
But, I can raise the hood!
The batteries in your phone only require 5 volts to charge, the batteries in your devices are 3.7 to 3.85 volts. 6 volts are plenty to charge your phone you just need to get the polarity correct.