Q: I want to install tachometer what do I hook to in the ignition switch?

Hi there. Take the power wire to the tachometer (the input wire) and hook it into the return wire on the coil system. This will register the tachometer and make it work. Put the ground wire on a good source of ground and then start the engine. The gauge should work. If you need further assistance with hooking up a tachometer, then seek out a professional, such as one from Your Mechanic, to help you.