Basically what you are doing by leaving the OEM wiring in tact is cutting in half the amount of current the two bulkhead wires have to carry.
The output post at the alternator and the post on the starter relay will both be at the same voltage. And the factory wires will be splitting the current to the welded splice (granted 1/2 the current will be going through the ammeter) and since the load on both sides of the anmeter will be the same it will not move.
You can move one of the connectors on the back of the ammeter to the other stud and effectively bypass the ammeter.
