You need to run a wire from the negative side of the coil to the signal input on the revcounter. Find a pos+ and neg - supply to run to the other two inputs on the rev counter. You may also need to run power to the bulb of the rev counter for it to light up at nght. Make sure the light doesnt stay on all the timeyou'll get a flat battery!
Use a wire that becomes live when you switch on the ignition.
Buy a test light from halfords or a car spares. They are well cheap an will save you loads of grief. Good Luck
Adz, July 2007
