VDO is the manufacturer of the instrument cluster (speedo etc.) - if you look you will probably see VDO on there.

I'm am not sure if 'behind the VDO' means in the engine compartment directly in line with the VDO, or means removing the VDO.

I've never heard of the deceleration sensor - what's it for?

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190E's:
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
http://www.maylane.demon.co.uk/190esmall.jpghttp://www.maylane.demon.co.uk/190esmall2.jpg