Heatmap for object detection model


I am using opencv dnn module to run a (yolo) object detection model.
This is working fine.

What i want / need to do is to produce some kind of heatmap for the class activations like shown here:

This should be possible with opencv. I am just not sure how and i could not find anything on the internet. For example i am not sure to which layer i should forward
Any top level explanation how to do this is highly welcome.

Thank you very much + Greetings,

Ok in the end i just convert my model to tensorflow and use existing tooling