If the connections are corroded, dirty, or otherwise nasty looking, they are suspect. A good cleaning will reduce resistance and allow proper current flow.

Starter changes aren't terribly tough. I did my 240D twice. The first time took over an hour, the second time about half as long. The 300D should be similar.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.