Car won't crank.
 

Started he up this morning & drove her out the garage to do some work on her.

This afternoon:

1) Ignition on all good battery ok.
2) Turn key & nothing. No dimming of ignition lights.
3) Won't roll start
4) Drivers side window stays extended when door open
5) Checked all relevant fuses & all ok
6) Removed battery terminal for 5 mins & reconnected...no change.

Got me stuffed atm

Classic symptoms of immobilizer damage (usually water infiltration). Your blown C3 fuse also points to this.
Feel under the seat for moisture. There is a fuse on the immobilizer that should be checked (disconnect battery), then check the electrical connections and the circuit board inside for signs of water.

Thanks I'll check it out.

Dry as a bone under the seats.

How can I confirm it's the immobilizer?

And if it is what's the fix?

You'll need to disconnect the battery to unbolt the seat and visually check the immobilizer and its fuse. If you do find signs of water damage, you'll need to find a repair shop in the Land of Oz or ship it off to the US (ECU Doctors, $500-ish for the repair).

Another possibility is the ignition switch, which is a cheap and easy fix. You should probably order or pick one up from your local auto parts store so it can be eliminated. It's a 10-15 min job and they run about $20 in the US.
It should probably be replaced either way as it is low cost and a common point of failure.

Are you depressing the clutch fully in neutral? If not, she won't crank. My 2001 Boxster S does this if I don't push the clutch all the way down.

To add to this, take the immobilizer apart and look inside for corrosion. It's just snapped together. There's also no need to remove the seat to get to it. I also agree with Charles that I would suspect the ignition switch second, followed by the clutch safety switch third.

Plugged in the Durametric & surprisingly no fault codes.

I'll get time tomorrow to check the immobilizer & the ignition switch.

Removed the immobilizer....top of circuit board looked fine....underside had small patch of water damage corrosion (green oxidization) which I cleaned up with metho & a toothbrush.

Reinstall & she starts up like normal.

Thanks for all your help & comments it really got me out of a pickle.

This forum is awesome.

Great news! If you get any other weird gremlins, you may need to pull it back out and check the connector pins again, cleaning with a brush and contact cleaner if necessary. ;)

Thanks I'll do that if required but hopefully it won't be required.

Cheers

Be sure to check your drain holes periodically.

Is there a way to pull the connector pins out to clean them? I took the connector out and scrubbed it, but not sure I got all the crap out... I replaced my immobilizer and got it resynched to the ECU (ECU doctors out of Florida)....

Now my windows wont work and car still wont crank. I have not replaced the ignition switch or clutch safety switch, but before doing that I wanted to make sure there wasnt some kind of issue with the connector... I have new pins, and might want to just replace them and repin... just not sure what to do, since it does look fairly clean now.

so frustrating