TOPIC: '62 Merc 500 Tach Hook Up?

Hi,
I found a thread where this was touched on. Looking for more details so I do this right... I have a '62 Merc 500 and an Air Guide 865 tach. Tach has a positive and negative terminal. So negative goes to any ground to he battery? Positive goes to where? Last project I hooked it up to the key switch, "M". Easy as it was in the dash and works great. This key switch is in the control unit. Do I need to go in through the screw pictured or do I run a wire all the way to the engine? If to the engine how should I bring it in the housing and then to what screw or connection? 4 pics of what I have. Has anyone done this? Pictures? Please mark mine up or post a picture of yours or diagram.
Thanks,
Eric

IF IT IS FOR your motor neg goes to ground on the mag and pos goes to the shutoff stud on the side of the mag 658 is the one you need was your #a misprint Airguide does not show a 865 in my search'

Hi Dave, If you zoom in over the white label it is '865' in black ink could be a misprint... Here they show a 870 (picture) and no 865. So I run a two lead wire all the way back following the control harness and into the housing best way possible? Guess I could try to trade for a 658 if it doesn't work.
thx

yes your merc 500 1962 has a mag. call me if you need. 1 914 310 7086

HUGE THANK YOU TO DAVE. SUPER NICE GUY, VERY KNOWLEDGEABLE, GREAT TIPS !
HERE ARE TWO PICTURES TO HELP THE NEXT PERSON OUTLINING WHAT WE DISCUSSED.

Hi Dave,
I might be in the market for a tach for my '77 merc 1150. It quit working at the end of last year. I did a little investigating this year but didn't find a problem. With the weather not cooperating this year I haven't had it in the water yet. Just wondering how available it will be and how hard a hit I am going to take. I would like to have a some what period correct one for my '59 Thompson - '77 Merc combo.
Thanks,
Bill