Car wont charge?!

I have a 1.3 mk1 that i have just put a new engine in and it wont charge, i have tried a new alternator and battery and now im out of ideas
the thick black wire from the alternator goes straight to the battery and the battery light wire goes to a turqoise coloured wire by the battery
when i put the ignition on the battery light comes on and then stays on once i start it. i put a multi meter on the battery and it reads around 12.5 after jump starting it then slowly drops and then the car cuts out
any help would be really helpfull  
Cheers!

as long as your ign light stays on it will not charge, you should have a RED wire from your alternator which should terminate at the starter motor and then continue to the battery,the blue wire goes via the ign switch to your ign warning light and back to the alternator, its this wire that controls your charging, might be worth checking you have the same battery volts at the alternator its self, also if you have a good earth to the engine and chassis. by the way a full charged battery should read at least 12.6 volt and when charging depending on load at about 13 to 14.5 volts, hope this is of some use to you

Thanks so instead of the thick wire going to the battery it should go to starter first then battery? I don't see how that would make a difference but I'll have to look into how the ignition wire is wired up because i'm asuming that must be wrong thanks for the help

You obviously just snapped the blue wire somewhere, during the engine conversion.

cheers for the help, but if it was snapped would the light come on with the ignition?