Wire brush the glow plug terminals. Resistance readings are difficult. Twist the connections into your meter and push down. I test from the positive terminal to the outside of the plug. No need to remove them. But unhook the power source obviously.

Floor the throttle 3 times before starting. Then wot while you crank.

Someone just posted an actual mercedes cold start procedure. For below 10 that was it I think. I'll look for it.