Ford 555d Backhoe alternator problem

I have a 1996 ford 555d backhoe w/alternator problems. The alternator light does not come on. The bulb is good. The alternator that was on the backhoe when I bought(not sure it was working), it did not have a thermistor plug but when I bought a replacement it had the thermistor plug. I installed it but still no luck w/charging the battery. I checked the W post to grd and it showed 0v. The people I bought the alternator say I have the problem and offer no help. Is there something between the alternator and battery that could be my problem?Thanks