I tried this in 1989. I put two 3.5mm sockets on the rear of my STe one for 5V and one 12V. The ST doesnt need much 12V power, but it eats amps at 5V. At the time the highest power 7805 I could find was a 2A version. It powered the STe for about 10 seconds before it had a brown out. I have recently secured some 5A 7805's in large metal cannisters. Some of these in parallel will work fine. BTW, dont use the 12V straight from the alternator, it can be 14-15V or more sometimes. Use a regulator.
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<SNIP> The fact is that ST need 12V supply as car could give. The only operation to power up a ST on a car is to remove the original power supply and replace it with some electronic filtre that would make alternator voltage straitly constant to 12V and a LM7805 componant to supply the 5V.
