blowing fuses for headunit

[mk4james]

Hi all 

So i went out and bought a new headunit, i got home with my current headunit still in the car working, pulled it out connected the new headunit, before i have even turned the key i could hear a bang in the glove box its done both fuses. replaced the fuses a few times and they just blow as i put them in. I've looked around the wiring to see if the constant wire is earthing anywhere, any how and its not visible, now the strange thing i put the old headunit back in to see if that still works and it don't blow fuses but don't power at all. 

I have a 51 plate focus 1.6.

Thanks 

James

[isetta]

"new headunit" , is this that brand new or 2nd hand? is it genuine ford or something else. if not Ford, are you using wiring adaptors to connect ford plugs to non-ford sockets on the headunit.  could the adaptors be wrongly wired?