For those with broken hold down screws - start by spraying the threaded part with good penetrating oil - Kroil is better, or PB blaster. Do that several times over a couple of days. Then you have to improvise given the space you have. You might be able to tighten a drill chuck onto the broken piece. Or use a screw extractor or nut extractor. They making needle nose shaped vice grips - that might fit.

I would disconnect the battery cable before doing any of this to reduce the danger. Do what you can to protect the battery from power tools (like slip a piece of sheet metal alongside the battery to protect it.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles