Thanks for the comments - no scope but I could find a frequency DMM without much difficulty.
I'd like to use a VSS (something independent of gearing) which means the VSS or merely the rear ABS signal would be useful. If I can't use that without significant filtering, a signal from the spark plugs would be used instead. Is there likely to be a problem for the ecu/abs if I tap this signal though?
The other thing is that I don't know anything about it, but research on data sheets from other sensors seems to indicate the amplifiers on many of them peak at about half the input voltage - and many use maximum of 6V input (meaning a 3V peak output). Is this enough? Should I isolate it from the cruise unit somehow?
The alternative to this is installing a second hall effect sensor at the rear mud guard bracket, next to the original abs sensor and use its signal instead...