[Editor: We love our vintage cars, but many 356 owners also love their modern electronic gadgets, such as GPS systems, cellphones, and digital music players. When used in cars they all assume that a 12V electrical supply is available, but the 356 (with rare exceptions) was built as a 6V car. While some people are satisfied with charging their gadgets from the factory 6V cigarette lighter, others are more comfortable having 12V power available. Here's how one owner created a 12V outlet in his 6V 356.]
Here is what I did on my 1961 T-5. I went to Checker Auto supply and bought a standard replacement cigar lighter (which fits in the 356 dash hole just fine) and I ordered a 6 to 12 volt inverter from J C Whitney. I removed and stored the 6 volt 356 cigar lighter. I removed the 356 cigar lighter knob and installed it on the purchased lighter (so the dash looks authentic). I removed the heater element from the 12V unit so it can never be pushed in and operated as a lighter (I am not sure the inverter can survive this resistive load).
I mounted the 6 to 12V inverter under the dash (they are pretty small now and fit in many locations. Next I wired up the inverter to a switched location on the fuse block. You can hook it up to a non-switched location but you risk the accessory (Ipod, GPS etc.) running down your battery if you leave it on after stopping the car.
These inverters do not like inductive loads like motors, etc. but are fine with modern electronic devices.
To protect the original lighter hardware I bought a replacement at the auto supply store.
Be sure to remove the heater element so there is no chance of a resistive load on your inverter.
Here is a physical schematic of typical hardware. 4V to 8V goes in and 12V comes out. The third wire goes to ground.
This is the 6V to 12V inverter mounted on my T-5 near the fuel valve handle pivot support.
My system is connected to the top right switched fuse block terminal.
Plug in your GPS, phone or I Pod charger and hit the road.