The only thing that looks like a “mushroom” and located behind the front tire, is the sub-frame (engine cradle) isolators. They can be ordered from GMPartsDirect.com., or purchased from a dealer. The part number for one is 14103535, and the other part number is 25536050. If you do the complete set, you will need 6 of each, 12 total! (Six upper, and six lower). I installed a complete set for a customer last month and my cost was about 8 dollars each. I installed them, one set at a time. Took about two hours. (No major rust on this one)
You may need the bolt and washers also if your car has been subjected to a lot of road salt. They eventually rust through and break. Hopefully, you can get the remaining stud out. I have seen the cradle rust through also. Bad news when this happens, a new one cost about 900 bucks and a PITA to replace. Remember, the cradle is the only thing that holds the engine & transaxle in the car!