I'm gonna take the lazy route today.
Step 1) buy and install an AC inverter.
Step 2) buy a bipolar voltage output power supply and place or install in a convenient spot in your auto.
Step 3) plug in and turn on.
** Note: you may need to iterate the steps, if you didn't purchase a large enough AC inverter or didn't use large enough gauge wiring.
You may also need to purchase a secondary battery and charging system depending on your needs.
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You can do that with an inverter. Inverters literally invert voltage, but the term 'inverter' has came to mean DC to AC converters for some reason. They are a type, or a class of switch mode power supplies, and have all the downsides of them. They can introduce nasty RF into everything!!! Emitted, and conducted through to the power supply rails and output! If thats fine, then use that. I can't think of any off the top of my head though. See what you can find online.
The virtual ground works by splitting the 12V rail, so that you have into 0V, 6V, and 12V, relative to ground, then you can declare the '6V' output as ground for your application, then, when those same voltages measured w/ respect to the newly declared virtual ground, you will get -6V, 0V, and +6V. I don't think that is what you want, especially since that means, relative to your device powered be this, the 'virtual ground' is 6V higher than EVERY other grounded object in the car, since everything else is still powered by a single 12V battery. That means you will need to be very careful about connecting up the load to the supply and not using the chassis of the car as a ground point. Remember it becomes the -6V rail as far as your load cares.
If this split supply works, and the current draw is relatively small, and care is taken to avoid letting out the magic smoke, then great. If not, an inverting SMPS may be your only option. As easier one to.
To make a extremely crude virtual ground, a simple voltage divider can be used. To make it's output of 6V relative to the chassis of the car more stable, adding filter caps across those resistors will help. This will work fine if you do not mind any meaningful current flow to cause the output voltage to swing all over the place. If the voltage needs to be rock stable at 6V relative to the negative battery terminal, Then, you can add a very basic voltage follower, to make the 6V more stable when loaded. You can use an op-amp for that, so that way, as long as you draw less than +-20mA, the 6V (rel. to batt. gnd) will not sag up or down. If more current is needed for beefy applications, then adding a emitter follower w/ complementary PNP and NPN transistors both used as emitter followers like this: You may notice that that is like a class AB audio amp. It basically is the basis of many amps, so you *might* be able to use a modded amplifier to do the job.
If you don't need MUCH (<20mA) and you can accept some noise, then the old ICL7660 or MAX1044 is a great chip
2 caps and 2 resistors create a virtual ground.
That kind of ground works only as a voltage reference and can't deliver / sink any (substantial) current. You could add an OPAMP as a voltage follower. But even then, it will +/-6V not +/-12V and still not be very powerful.
Would be easier to answer if you tell us what you need the -12V for...