Severe voltmeter needle oscillations

French demon said
But after a very short time, the needle begins to oscillate rapidly between 8 and 16 volts.
When I engage the reverse gear (which turns the reverse lamps on) or put the turn signal on, the needle falls completely to "0", indicating (I think) a short-circuit.
And when the needle indicates "0", I have no turn signal, no windshield wipers (but the headlights, flasher, stop lights or horns work)
I tried to unplug the reverse gear switch but the problem remains.

What car and year are you working on?
What modifications to the electrical have you or a previous owner done?
as asked before did everything work correctly before?

a few things come to mind...

The volt meter needs a positive source of power ( the fuse block assuming you hooked it to the right switched fuse)
The volt meter needs a good ground (The AMMMETER wires are BOTH positive, one comes from the alternator and one comes from the battery, some stops along the way for both) co a good body ground is what is needed here
when you engage reverse or turn signals, the volt meter goes to 0 makes no sense

JUST A WILD *** GUESS HERE, At this moment I'm betting when you bypassed your Ammeter you disconnected the 2 wires from the ammeter and did not attach the two together (typically one red and one black, both are 12V feeds, assuming this is 62 through 70 something before they went to the external shunt type Ammeter) That means that the car is running on battery or alternator only and some feeds might not be getting power correctly.

LASTLY go to MYMOPAR.COM download your cars factory service manual and wiring diagram and maybe go to CLASSIC CAR WIRING and buiy a laminated colored wiring diagram for your car. THEN learn how it all works and start tracing wires from the battery to the lightbulb for instance, and from the alternator to the same light bulb.