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.

The lights on all the time help keep moisture out of the housing.
when I first installed mine, a little moisture fogged up the inside of the lens, after just a few minutes the moisture was gone. Had to be the heat from the bulbs.

I guess it's cheaper to buy some bulbs then a new tach :-)