I've got a 1985 Mariner 70HP that I want to get a Tachometer for. Is there anything special I need to look for when shopping around for the tach? My Mercury Quicksilver control box has a pass-thru plug on it for the tachometer, but I want to try and wire it directly to my harness (I'm doing a restore on the boat, so I have a brand new harness and easy access to it).