The bolt holding the housing onto my 91 Bonneville fell into another dimension through the portal hidden under my car. So after 2 days of searching I have given up, problem is I still need to attach my housing. Need to know the size and thread pattern of the bolt, because apparently this knowledge has been lost to time.

Welcome to the forums!
Darn I hate it when bolts go into the abyss, never to be seen again.
May I recommend that you give a GM dealership, parts department a call.
They should be able to provide you with the exact bolt specs.

Measure depth of the hole, add thickness of flange to get length, and subtract 3/16 from length.......my guess is a 10mm thread....or just get a 10mm bolt and hacksaw to correct length....

Double check hole opening.....measure thread to thread, not hole opening.....

8mm=0.315"

10mm=0.39"

12mm=0.47"