So far, this is the only known online pictures or footage of Mystery Mansion at Indiana Beach (thanks to foch41). Skip along to 1:57 for footage of the ride.
Tom Spackman, Chief Executive Officer of Indiana Beach, designed and developed the Mystery Mansion ride in 1969, and it ran until 1998 when it was re-themed into the Den of Lost Thieves by Sally Rides.
You can also read a lot about pretzel dark rides here.
Details and information:
I’m currently working on a wikipedia article with all the info I could find here: https://en.wikipedia.org/wiki/Draft:Mystery_Mansion_Dark_Ride
If you have memories, pictures, or video, PLEASE link them or upload them somewhere and drop a link in the comments. If you were a ride operator, maintenance, remember any of the scenes or interior, please comment on those too!