Karmann konnection here in the UK do a voltage drop relay for 6V componants in 12v cars. These are used for wipers, semophores etc.
If it's a fuel gauge that you want, you can sell these electric gauges on samba should be worth a fair amount (especially if it has sender), then buy a mechanical gauge ebay or VZi.
In the UK these were an accessory for pre 1963 cars, but are rare with sender
I wouldn't wire the sender up as you burn it out, especially where it is as well.
You can never have too many accessories on your car. 🙂