If it the similar to those on my '87 300TD, they are rather simple bolt retainers that snap into receiver holes. I believe the holes are actually rectangular. Not certain, but they certainly don't appear as if they would be expensive to purchase and they are a breeze to install.

Someone else more familiar with your model may be able to correct me if I'm wrong.

__________________
Steve
'87 300TD - 132K - Soon 4-Sale
'84 300D Turbo - 122K - Driving
'77 VW Type II - 77K - Restored
'08 250EX Ninja
English Bulldog (Brier) - My best friend. Passed away 12/02/04 while in my arms.