Cleaned up the battery terminals and the negative cable to chassis connections. They were in "How was this even running?" condition. Brake wear indicator light comes on and goes off as it should now. Voltage at idle is still low, but I think that's because:

The glow plugs are stuck on.

Is there a quick way to enable and disable the plugs? I need to use the car all weekend and I don't have a spare glow plug relay to put in. Also, where is the glow plug relay?

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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 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.