Inside of number 1 are the test connectors for the glow-plugs.
You can check your glow-plugs with an ohm meter.
I recommend picking up a copy of "HAYNES" M/B 123 manual at the auto parts store. Good pictures and easy to follow steps.
I swapped out my rear axles using this manual. It took longer to jack the car; remove the wheels and drain the diff., than it did to change the axles!!
Good luck, John F.