'99 Mazda 626 tail lights out

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As you probably already know, the fault is usually in the rubber boot (already mentioned, above) connecting the boot lid and the car body.
Nothing to do with the wire going t the lock, its just the movement that causes the problem.

But you are almost there, when you know whats blowing the fuse.
 
I was right, it didn't make sense. Disconnecting the tailgate harness and the problem remained. Seems to be that when the boot is open, the boot light comes on and that seems to prevent the fuse blowing. Turn it off and pop. I assume that the lamp is pulling the battery voltage down just enough to keep the fuse intact. Of course, start the engine and it's game over. Went back to the circuit diagram and there's a joint box after the TNS relay. Presumably I can use this to isolate the sections of the circuit and narrow down the fault. If only I could find where it's located. The relay is in the drivers footwell. Can't follow the cable as it's a rat's nest under there.
 
To the best of my knowledge, the boot light coming on, cannot prevent the fuse from blowing.
The voltage drop because of a single bulb coming on will not cause the fuse to blow, or not to blow.

Hope this helps ..............

Did you strip the wiring harness going from the car to the tailgate?
 
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I wouldn't have though that either. However, the resistance of the TNS circuit changes between 8 and 1.2Ohms with the operation of the boot light (it has a switch on the side of the lamp). My theory (which may be apocryphal or at least, wildly inaccurate) is that maybe the short isn't making a very good connection and so has quite high resistance, so with the voltage drop of the lamp, allows it to fly just under the tipping point of the fuse.

Where the tailgate hardness pops out of the rubber boot and into the main body, there is a connector that plugs into the harness running along the roof of the car. I pull the two apart but it didn't help.

According to the circuit diagram, the TNS relay feeds into the jointbox where there are separate feeds to the two front side lights and a third feed to the rear. Hopefully I can disconnect these to see which is playing up.
 
Seems to be that when the boot is open, the boot light comes on and that seems to prevent the fuse blowing. Turn it off and pop.

We came across a faulty switch in the boot light unit, as you switch it off the switch tips the earth point and pops the fuse
 
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