[Metro Mighty Mouse]

Okay, finally found the necessary diagram in the factory manual (same items may be in several diagrams and I forgot to look for the DRL breakdown. I was just looking at head and tail lights diagrams). If everything works with the key off, and key ACC, and key on, and the fuse only blows with the car running, the only thing that makes sense (95% sure) is the DRL module has an issue (it has a sensor to tell it when the charging system is active. I am not seeing how it would tie into the tail lamp circuit but it is the only thing I see that should change from not running to running). If it blows when the key is on, car not running, I am somewhat less certain. In the grasping at straws category, try swapping the light relays, one is for tail and the other for head.

You are using the ohm meter correctly. A short to ground would show 0 resistance or an extremely small number. If you are checking a circuit through a bulb you should show a bigger number as you will be measuring the resistance of the filament in the bulb.

The key to your issue is the fact that it works until the car is on (or running). You have to look at what is changing in those states and what in the list of changes could possibly short out the tail lamp circuit. The most likely suspect is the DRL module.

The funny (sad?) thing is, because I was pouring over the DRL diagram, I finally saw the weakness in the wiring layout and the proper solution. It turns out that the headlight relay really doesn't reduce the current through the switch, it is actually wired in after the switch. It appears the most economical place to add relays to the circuits is at the junction block with wires in the same plug as the brown/yellow wire. The red wire is the high beam wire and the red/green wire is the lo beam. You would use a single relay on each wire (2 total) with 12v into the coil (#86 on the relay diagram) and the other end of the coil hooked to the wire going to the plug (#85 on the relay diagram). Then the wire to the headlight connected to the switched side of the relay (#30) and terminal #87 connected to ground. This layout should make the headlights brighter as the path to ground is shorter and will prevent overheating the wiring through the junction block and the headlight switch as they are only carrying enough amps to switch the relay. This also has the advantage of putting the relays out of the elements and away from engine heat (not generally a problem, but failure would be less likely).