Insurance companies are the worst, especially in situations like this. Are you dealing directly with the other person's insurance company, or with your own insurance company (who are, in turn, dealing directly with the other person's insurance company on your behalf)?

Can you ask your insurance company for advice?

In terms of establishing a value on your own, start with the NADA guide:
1984 Mercedes-Benz 190D 4 Door Sedan Value, Prices & Specs | NADAguides | NADA 1984 Mercedes-Benz 190D Book Value & 1984 Mercedes-Benz 190D Market Price

Also, search eBay for recently-sold examples:
mercedes 190d | eBay

Autotrader:
Used Mercedes-Benz 190D Vehicles for Sale in Worcester, MA 01604 - Autotrader

Cars.com:
Used 1984 Mercedes-Benz - in 01604 Area - on Cars.com

I'm sure there are other ways to corral numbers, but that's the best I can come up with. Best of luck.