Cars and Automotive Design

hi guys. bit of a tricky 1. have had my designer create a 3d rendering of a vehicle and added our kit design to it, iv been sent the finished design In stl format which in return I can open no issue but lokking now to get the vehicle scanned so I csn scale to sizr and do the reverse design (bumper attachment points etc and also separate the rear diffuser add on from the bumper. I cant seem to select these parts obviously separate but can easily see the body lines, the issue is the mesh lines etc . I want to split the parts to separate parts off the vehicle if possible.

thanks guys