Create the call to sub function at the place where pin of Mikrocontroller is getting set.
In this case the rpm Signal is like "redirected" cause its in an pinchange Interrupt on mcu. There you have to Check if you are showing mapswitch or not. If yes just return nothing and let pin untouched.

Then create a another Funktion on 1ms Raster and to "rpm to Hz" convertation.

Take a Look in schematics of ecu, you'll find them here. It should me P1_13 or P1_12. Or something like that.
I dont remember that now.

I got some Videos while reversing it with ols300 and oszilloskop.

Regards Benni