Fuse 23 keeps popping

[Lurch320i]

This isn't for my car but a mates
Everytime he puts a fuse in it pops imediately, doesn't matter if the ignition is on or off and it's not runs out soon so needs it sorting
Any help will be appreciated
Thanks in advance

[Grrrmachine]

Probably the rear number plate lights have gone rusty and started shorting out.

[Lurch320i]

Cheers grrr but even with the lights and ignition off it's still popping

[Lurch320i]

Excellent thanks
I'll tell him and we'll try that tomorrow

[Lurch320i]

Right
We unplugged the big c103 plug and it's still popping the fuse
And there is also another purple wire behind there that's tapped into something with it's own fuse link

[Brianmoooore]

With the C103 unplugged there is only about a foot of wire connected to fuse 23, which runs down through the duct in the bottom of the fusebox, through the bulkhead, and into the socket, unless, as Will has already suggested, the bodgers have been at work, and something has been connected to this wire that shouldn't be there.
Is the purple wire connected to the grey/green wire?
Is this a car that has had an engine conversion?

[Lurch320i]

Originally it was a 320i and now it's a 2.7block with a 2.5 head

There's an alarm on it and when the wires are wiggled under the coloum the alarm goes off
The wiring is a mess behind the dash
We have the radio and heater panels out clocks and gonna take steering cowling off to try and make sense of all this
Gonna follow all the wires bak and see what does what
Thanks for the help though

[Brianmoooore]

Who did the engine conversion - was it done before the car was acquired, or did your friend do it personally? Was there any smoke from the wiring related drama the first time the ignition was turned on?
If so, or if you don't know, find the main engine loom plug and socket (C101), mounted just inboard of the fusebox. Take this apart and slide back the rubber boots on both halves, then check the colours and condition of the wires connected to pin 20, and report back.

[Brianmoooore]

Latest on this problem, received by PM: "the problem with the lights has only just started I have found today that if I have the ignition on and insert a fuse it does not blow but when I turn the ignition off it blows or if the ignition is off. When I insert the fuse it blows straight away the purple wire with the blade fuse in is for the remote alarm and locking system I think a wire must be touching the ignition live"

It looks like the fuse 23 is being asked to supply power to the entire ignition circuit by an internal fault with the immobiliser. Try temporarily fitting a 15A fuse in position 23 with the ignition already on, then starting the car.
Turn the ignition switch off and see if the car continues to run. If it does, turn the ignition switch back on quickly and remove fuse 23, before switching off.
Still have to work out why an alarm/immobiliser would be connected to fuse 23. A failed attempt to flash the parking lights when setting/unsetting?