hi, i recently replaced the rear speakers in my 98 camry but i noticed they have proprietary mounting (3 screws..) any suggestions?

Get your money back and find speakers that fit.

how did you get the nut that holds the safety belt anchor off? it seems welded on when i treid.

What size were the replacement speakers? If they were 6x9, most aftermarket speakers have 4 mounting screws. You'll need to drill new mounting holes but the improved sound quality will be worth it.