Land Rover Repair: Alternator input drops with electrical load, fog lamps, fuse box

Question
I've got a 1998 Discovery 90k, new plugs and wires. Tach was acting erratic, changed the alternator and the battery. When I start adding and electrical load the tach drops off and so does the output from the alternator. At Idle with 750 rpms output is 13.9-14.VDC with input to the alternator @ 13.4 DC,  lights turn on I see the tach drop, and return output voltage is good, i start adding more load fog lamps, defroster, tach drops below 500, output from alternator goes to 12.6 DC and input to alt. drops to 12.4 and dropping as load increases. Engine runs fine, just that drop in voltage. Where do I start?

Answer
Either the alternator you installed is defective, or there is a main power distribution problem; i.e. a failure in the fuse box or power cabling to and from that and the alternator.

You would be surprised how many so-called rebuilt alternators are just polished junk