I would take the plastic piece off the trunk and figure out how to flatten that metal portion that has been puckered from that rear end hit.

probably some hammering is in order. then, I would re-paint any rusty metal underneath the plastic with some sort of rust paint (like POR-15 if you have it, regular auto spray primer/paint is fine if this is not critical). then put the plastic back down with a bead of silicone underneath it to fill any remaining gaps.

hope this helps, should be a couple hours and $20 worth of materials to fix right up

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car