Head unit not powering on

[bob540]

Not exactly mini-specific but I'm sure you all will be able to help. I bought a new head unit a couple of days ago, tried fitting it and it's not powering on.
The old and new unit have the exact same wiring diagram for the ISO connectors so it should've just been a straight swap but there's not a kick out of the new unit and I put back in the old one to check if it was a faulty unit but the old one doesn't get power now either.
I used a phase tester to check if the wires were getting current and all seems perfect, the constant is getting constant power, the ACC is getting power when the ignition is on and I think I ruled out a bad ground by using the phase tester to create a good ground by sticking the phase tester to the ground part of the ISO connector and clipping the alligator clip on the other end of the phase tester's wire to an unpainted bolt attached to the chassis (which I used as a ground when testing the live wires and it worked perfectly) I checked the fuse in the back of the head unit, the fuse box under the bonnet and all of the in-line fuses behind the air filter and they're all fine (am I right that if a fuse was blown then I wouldn't get the phase tester lighting up anyway cause the connection would be broken?).
My head is wrecked trying to figure this out so any help here would be really appreciated.

[irlmin]

Some head units have 'internal' fuses as well , you may have to open the casing and have a look , BUT beware if you go down that road you may invalidate the warranty if its a new unit , I would also think you should try to introduce a positive feed to the headunit bypassing the 'block' connector and see where that goes -- does the unit power up if so you know your problem is the vehicle ,if it does not it is the unit .
G

[bob540]

Well I don't think it's the head unit because the old one isn't getting power either. I got a multimeter today and checked the wires and both the constant live and acc wires are getting just over 12volts so I'm stumped!