You might try using an endoscope -- one of those tiny cameras that works with a phone. I once used the unit linked to below to find a bolt I'd lost in the headliner. Worked great.
Depstech Wireless Endoscope, WiFi Borescope Inspection Camera 2.0 Megapixels HD Snake Camera for Android and IOS Smartphone, iPhone, Samsung, Tablet
Depstech Wireless Endoscope