I tried to download the test proceedure from the manual but it would not down load.
A single Glow Plug cannot take the full battery voltage and the loop/filiment/heater element will instantly burn. Apparently the resistance wires reduce the voltage to a managable level.
Unlike a Spark Plug or the later style pencile Glow Plugs the loop/filiment type Glow Plugs are not grounded on the Glow Plug Body. The Electrical Current passes through the Glow Plug, heating Element and back out to one of the wires or resistance wires and the circuit is eventually grounded at the very front by a single ground wire.
As eletrical current passes through each Glow Plug there is a voltage drop; the possible amount of the drop is listed on the testing diagram.
For more testing info look here:Repair Links
Fast navigation
http://www.peachparts.com/shopforum/diy-links-parts-category/146034-fast-navigation-do-yourself-links.html
Assuming your pushbutton switch can handle 80 amps you are a really good canidate for the updated pencil type glow Plugs. Mercedes part 0250201044 Bosch number 80035.
Those Glow Plugs get hotter faster and people who have change over to them are seem very satisified with the change. See the Repair Links on how to.
Should be pictures in the below thread.
http://www.peachparts.com/shopforum/diesel-discussion/378463-glow-plug-upgrade.html