The ABS Control unit (ECU) is also behind the battery, but usually has a very large multi-pin connector along the top, and no 1/0 switch.
I've never seen one of these MAS relay boxes before though, and would've been pretty confused by it!

later

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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