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Anyone able to tell me what part this is & if it's replaceable by chance? Also, how screwed am I exactly?
This is where the rear passenger trailing arm attaches to the body & the cavity where the screw resides.
Many, many thanks!

 
Rep Power: 229

Looks like you have enough good metal around it to work with.

Worse case scenario you could find a solid example of your car at a scrap yard and cut out the peices you need, then weld them back into your ride.