I had a similar issue of noise interference on the supply line to my speedo.
I built my car with an electronic Autometer speedo and the BMW sender in the diff (which is a reed switch)
Eventually, I solved the problem by adding a 47nF capacitor across the supply lines close to the speedo. After that I had absolutely no issues.
Full details here...
http://www.madabout-kitcars.com/foru...ghlight=speedo
Try something like this from Maplin....
http://www.maplin.co.uk/p/multilayer...apacitor-dt97f