I managed to swap out the pins in the extension cable using a sewing needle. Pretty easy actually!

I hit the power button and it lit up for a split second then went off again. There was also a short pop from the speaker like it was getting power. The red led on the mobo flashed on for a second but isn't staying lit.

Now to figure out what's stopping it from sparking up the whole way. There's no PRAM battery, could that be the cause?