Yay!!! I thought you'd get it today! Good to hear the Law of Sod is alive and well and waited for you and Bob to go out before sending the driver rond with it.
I've had a look for a 1990 240 diagram but can't find one Alan, it would be helpful to see photos of where the voltmeter is fitted on that one you refer to as i think (but might be wrong) it's meant to be integral with the instruments and should therefore pick up the feeds from the instruments with no other wiring involved.
He's obviously got a backfeed somewhere, perhaps on the bulb killing the tacho signal.
http://www.myvolvolibrary.info/Tech_...g_diagrams.pdf