1. You can identify two 12V positive rails by a label on the PSU that gives two separate 12v load limits for output 1 and output 2, and by observing that the +12V wires coming out of the PSU are connected to one of two distribution points on the PCB instead of just 1.
2. The trouble with using auto bulbs as loads is that they take a very high surge current when you first turn them on. It may be this surge current that is tripping the PSU.
3. Another reason for the PSU tripping may be that you don't have any load (or too little load) on the 5V output. Loading the 12V rail on many ATX PSUs will make the voltage on the 5V rail rise unless it is properly loaded, and this may trip protection circuitry.
2. The trouble with using auto bulbs as loads is that they take a very high surge current when you first turn them on. It may be this surge current that is tripping the PSU.
3. Another reason for the PSU tripping may be that you don't have any load (or too little load) on the 5V output. Loading the 12V rail on many ATX PSUs will make the voltage on the 5V rail rise unless it is properly loaded, and this may trip protection circuitry.