Annoying fuse-blowing problem : Mk I Punto

Hi folks,
I'm trying to fit a new head unit to my friend's mark 1 punto. It's a CD player made by "Shark" which I bought from Maplin. (it was on offer!)
When I first fitted the CD player, nothing happened, and I assumed it was dead. But then I noticed that some other stuff had gone off too - interior light, clock, etc., and traced the problem to a blown fuse.
Thinking it was just because I hadn't disconnected the battery when I fitted the stereo, I replaced the fuse - and got my fingers burned when it popped again!
So I took it back out and got my multimeter out. It turns out that the power connector in the car and the power connector on the head unit are differently wired - notably the constant 12v supply, and the 12v supply from the key barrel were the opposite way around. Also, there's a white "auto. ant." wire - which I presume is for automatic antennae. I disconnected this, and swapped the other two.
When I fitted it again, same problem - blown fuse.
I've noticed that, on the new head unit, the speaker's ground is also the power ground. I haven't investigated further to work out if this is the source of the short.
Before I go any further, is this a common problem which I can buy an adaptor for or something? Anyone else had similar problems?
Cheers in advance!
Adam.
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the sort of connector the punto has fitted as standard is a regular iso conector.
http://www.maplin.co.uk/media/largeimages/19651i0.jpg
like this. what sort of connector did this radio have built into it? i found it on maplins but i cant find the manufactures website. not sure what else too say, does the fuse blow when the head units not connected?
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