OK, I installed the new positive terminal from Pep Boys. Then, a flash of creative thinking came through my brain. I pulled the 1991 560SEL out and pulled up in front of the diesel. I then hooked up jumper cables from the 560SEL to the 300DT. I started the 560SEL and revved the engine a bit. I then tried starting the 300D. It turned over fairly well (before it wouldn't turn over AT ALL) which tells me it may be a shorted cell in the 300D battery.

Question. How can I test for a shorted battery cell in the 300D?