...To keep the Bolt Heads from Turning, Slide a Square Bar of Steel, Long Enough and TIGHT under One of the FLATS. To keep the Small Bar in Place, Use Tape. The Piece of Steel could even be Tapered, to slip in and Tighten.
Or, You can reach in with a Small 'Flat Tipped' Pry-Bar, and JAM the Bolt from Turning. And, ofcourse, there's always the trusted 'Box-End' Wrench or Socket. The last two may require a helper under the dash...or tape the wrench in Place.
Personally, I used a Different Approach, that many would Never Do.