aftermarket tach ??

[84socal]

ok i have a sunpro tach i got out of a different truck i had and set it on the shelf for a while and now wann thow it in my stock crewcab but kinda unsure where to hook the signal wire to

my buddy says it hooks to the - wire on the hei plug the the rest is power ground and dimmer
my question is i looked at and hei dist. that i had sittin in my garage and it has a tach out put and looks like a plug goes in there
and the one on the truck looks the same but im wonrdering if need to find a plug is it already in there can i attach the wire to somethign else im new to the gm stuff and i know toyta 22rs really well so any help would be appreciated and if any one has a pic or 2 the help thanx in advance