Well, if it's blowing the fuse immediately even when both light switches are off, then I think you could assume that there is a short in the wire that runs from the fuse box to the light switch.
On the other hand, if it pops the fuse as soon as you turn a particular switch on, then it would point to a short in the wire from that switch to the light, or in the light fixture it's self.
Insulation can chaffe off the wire anyplace it runs by something metal. If it wasn't blowing fuses before final installation of the outside light, then I'd look in that area first.
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