Realised I may need to add a spacer at the front of the upper collar to allow the front blind spot cam to be hidden - no slats...! 

Beautiful. Brilliant work. 

If it is of any use to you, the screenshot is a modified version of the Mk6 audio stages. It includes a power regulator, hale rail supply for eth op-amps, a tone circuit and an output op-amp with a volume pot. It's not complete but maybe it will get you started. The output op-amps has two capacitively coupled inputs, one is the modulated audio, the other is for an auxiliary input like mp3 player or something. You can get an op-amp in a quad package so you can do all of this in one IC. 

Hi Kev. Sorry it has taken me so long to respond. I don't have much spare time at present. So overall, the modulator works well. If you have the vocal skills (which I don't have), I imagine you could make is sound really great.  The odd time I made it sound quite realistic. Some technical comments for you (my personal opinion only):   1. Because it is a chopper design, the duty cycle doesn't do a whole lot for you. Varying it doesn't seem to do much as far as audible output. 2. The mic volume seems to not do much, not sure it that is me not using it properly but it seemed to have little effect. 3. With no mic input, I can hear the LFO out the speaker. In the time I had I couldn't work out the circuit enough to offer a design change, but I would maybe look a a sound "gate" that only enables the audio put when the mic input gets to a set level. 4. You used a kit type preamp, which is great but that kit is designed with lots of different uses in mind, hence its size and complexity. I would design an op-amp circuit made specifically for your units, which could be very small and simple. If it were me, I would design a small module with an input and output op-amp onboard and a connector in the middle where you insert the modulator you are currently experimenting with (or supplying). In this way you take care of input and output impedance, level, and I would go as far as building in a low pass or band pass filter to keep out unwanted noise. With its own power supply onboard (78L09 or 78L05 maybe) you can make sure it has a nice clean supply separated from the modulator doings. I would possibly put the output volume control as part of this board, and if you really got carried away, a tone or bass and treble controls.     Thanks for the opportunity, I'll get it on its way back to you this week along with a couple of scope probes I got given that may be of use with your scope.   Cheers, Glenn.