Hey Mouse,
Thanks for your help and push in the right direction
Here is my small contribution, Just received the final piece of the puzzle today, hope to do some testing in the weekend
So I spent a whole weekend reading up on how to build this, most of the people that made this sort of hertz generator did not specify what kind of frequency range theirs had, and I think theirs were for other purposes, like running speakers, and they looked OTT for this job, so needed to get into the nitty gritty of it
I may post some pics and build data if needed
Looks like I can do some much better testing now, I think I may have gone wrong somewhere with the other means of testing, maybe a bad signal feed from the car?, or just my incompetence, I think just me

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this is a little bit of an elaborate build, but it can be simplified into 1 potentiometer , 1 Resistor , 1 capacitor , 1 frequency panel meter , a 555 timer , a small circuit board , random wire (maybe total $25 NZD), If you are lucky to have a multimeter with inbuilt frequency/hertz read out built in it, it could cut this cost in half....
will post some more about this on this thread if wanted.....
Good Day....