Immobilized - Radio Swap

[78F350]

Originally Posted by Coldbourne

... From what I understand, leaving the key in the ignition for over two minutes will trigger the alarm immobilizer. I may be incorrect, but it is what I have read on the forums. ...

What forums are you reading? I've never seen this stated here, never heard that about our 986s. Give me a link, I'd like to see what you are talking about.
I'm guessing a fuse, or relay is your issue. Back up and start troubleshooting from the beginning without the assumption that its the immobilizer ***unless you have some sort of aftermarket alarm system hidden in there somewhere.***

[pachinnery]

I have just changed the stereo head unit in our Boxster 2001 2.7 manual. I left the key in the ignition for at least 15 minutes and didn't experience any problems. I have had keys fail thought. The key is made up of three parts blade, remote control to operate doors/alarm and immobilizer tag. I would guess the electronics to communicate with the key fob when the key is in the ignition must be below the steering column. Maybe you have disturbed this wiring in fitting the stereo. have you tried the spare key?
I hope this helps.
Regards
Paul