Ok don't shoot me down as I know naff all about car electrics, but I was getting a similar problem as you I also changed the alternator which didn't work, it turned out to be a bad cable joint on the + feed to the battery, if you follow the + from the alternator to the point it meets the wiring loom on the inner wing strip back the tape and you will find a crappy solder joint that ford have made to step up to a larger cable which corrodes, it's worth replacing the alternator feed to battery with a new larger cable which helps with voltage anyway,
you can do a quick test by using a jump lead as a jumper wire from the back of the alternator to the battery and see if this helps.