No power at key
OK, let me confirm, you checked last:
One probe on the battery positive
other probe first on one, then the other ammeter terminals
AND lights are on "and dim" and it shows zero volts?
and still no power things like heater blower, from the key?
If this is so, we need to confirm a couple of things:
THIS TIME clip one meter probe first to one, then the other ammeter post
Clip the other to a good ground.
Confirm that lights are dim, and measure this voltage
Try the key in accessory, run, start, while watching the meter, and try accesssories like wiper, heater blower, etc.