190D Glow plugs not working

[KBCB]

Hi all, I was hoping to get a few questions answered. I have a '89 190D. recently the glow plugs have stopped turning on. I suspected they might not be coming on when the usual hum and relay click were absent when first turning over the key. Today I got the meter out and only got .5v at the connector. The plugs themselves are only 3-4 years old. So I don't think it should be them.

Does anyone have a wiring diagram and locations of the entire glow plug system?

Are there any relays that are known issues?

The battery that is in the car is just about at the end of its life. It will still start the car, but does need to be replaced. Would it make sense that even though there is enough charge to start the car that there isn't enough charge to fire the plugs?

Thanks in advance.