How to test zener regs before installation on 12V battery:
==========================================================
Starting point: a PR2 bulb and 10 Ohm resistor parallel, each
side connected to a zener diode which is connected to a battery
terminal. Each zener is embedded in a ring lug for cooling.
Both are 5W types, one is 6.2V the other 6.8V. Resistor is 1W.

Necessary tools: variable power supply and multimeter.

Turn the power supply to 3V and connect the zener reg
if the bulb lights up, you have the regulator backwards
connected. Swap + and - and proceed.

Turn the power supply up to 9V
If the bulb lights brightly then you might have the 6.8V
zener backwards, so it is a diode (0.7V instead of 6.8)
If the bulb lights not so brightly then you may have the
6.2V zener backwards, so both zeners drop 6.8V + 0.7V =
7.5V, which results in 1.5V across the bulb, measure 
which zener is wrong and repair (turn it around or use it
for another regulator at the opposite battery terminal)
before proceeding.

Turn the power supply up to 15V
The bulb must be lit now, else the bulb is broken.
If the bulb lights very brightly then you may have two
zevers of 6.2V in the circuit instead of one 6.8V, so 
the bulb gets 15 -6.2 -6.2 = 2.6V instead of 2V.
Likewise if the bulb is rather dimly lit, there may
be two 6.8V zeners in the circuit.
Measure the zener voltages to verify they are 6.2 and 6.8V
so the lamp gets 2V.

NOTE: with the lamp lit, the circuit should draw up to 0.7A
and the zeners get warm quickly, so work quickly or reduce
Voltage while preparing for a measurement to avoid 
over-heating the zeners.
In the final installation the battery terminals will cool 
the ring lugs that guide the heat away from the zeners.
Note that in particular the 6.8V zener is working near
its max rating when fully bypassing up to 0.5A through the
lamp and 0.2A through the resistor = 0.7A, resulting in
6.8V x 0.7A = 4.8W power. If your pack is unbalanced then
some regulators may see up to 16V which can push zeners
over their max ratings.

If you measure a zener to be a short circuit, then it is
broken. This is the normal failure mode of a zener,
especially when it overheats. (Internal meltdown)