I had a question on hooking up a hood tach and no answers and after racking my brain it hit me that the Tach wire is pulse and you need a 12 volt Ignition source also. I had hooked up the TACh lead on the HEI, a GROUND and the Wire for the lights to a spare light spade on the fuse block. The Tach needs a 12 volt power source to work with the pulse so I took the wire off the LIGHT Spade and put it on the spare IGNNITION spade and WA-LA the Tach works. Now I'm not crazy having the light bulb on all the time but at least I'm not buying a 250.00 tach for nothing. Hope this helps someone in the future.

This is on my 67 Firebird with an new HEI engine harness.