My suggestion was to have two sensors on opposite sides of the garage (eg in the vicinity of the present switches and using their wiring) connected in parallel to the light you have. Whichever entrance you go in will trigger the sensor near that door and turn the light on, so I thought that would do what you required, if not then I haven't understood correctly.
There was a thread here on a slightly different issue, but it contains the basic circuit you would need (ignore the labels on the diagram - each switch represents each sensor: , ie the switched output of each goes to the light.
, Is it OK to connect a PIR in parallel with a wall switch?, DIY Electrical Advice, ElectriciansForums.net Est.2006 | Free Electrical Advice Forum and page_number.
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You should be able to work out how to connect the sensors from the principle illustrated in the above link, otherwise I suggest you get an electrician involved,