You said shock tower...do you have hoops or factory towers? If you have hoops the rear plate may have been welded right over the VIN. If you find something stamped there but still cannot read it, try scrubbing the area with a dry (no water) brillo pad impregnated with pink soap powder. The soap gets rid of the grease/dirt and color-fills the VIN at the same time, so it is easier to read.
-aaron