probably a stupid question but i wanna be sure before cutting up my ecu

the grey wire that says its the rev counter wire, says you connect to pin 9 or 21 in the ecu
it then says that the grey wire always connects to the pin that you fit into the ecu multiplug
21 for non air con cars or 9 fo cars with air con.

what i dont get is pin 21 is the air con input into the ecu and pin 9 is blank
as i dont have aircon i dont have anything connected to pins 9 or 21 in the ecu.
and from the wiring diagrams ive got nothing is connected to either of them anyway, and the rev counter takes it feed from coil/ign amp

any1 know what im supposed to do?

will call msd later but they aint open yet.