Thanks very much for your reply numb-nuts.
You raise some really good points. I will start having a look around for some replacement mirror glass for these mirrors and then i can drill them without damaging the original ones fitted to the car which could be reinstalled at a later time.
I also think the extra slow drain battery is a very good idea, I will have to look at how easy it is to run power cables from the engine bay into the cabin of the car to power the DVR/Cameras.
Thanks again for taking the time to reply, you have been a big help.