Vintage Synth Repair

Involving circuit bending, that is. It was actually very easy. I found
two points by trial and error where I could somehow patch the out
signal of the sound chip to the cutoff point where the pot for the
cutoff adjustmens is, soldered two wires and a switch on there And now
it screams!:)... It actually works somewhat as a ring mod with the
resonance all the way up. I'm pretty darn sure that others would have
done that differently but it works just fine the way it is. But now I
want to add a audio in but where would you suggest to put the wires? I
would rather have the external audio signal go throuth the sound chip
before it goes through the filter and I need to find a way to put a
switch in there to cut off audio signal, so that I don't have anything
else going with the audio signal. Can anyone help me out with that?

PS: I'm still looking for a way to increase the LFO amount in depth
and width.