I had the same problem with my SD. The dealer wants darn near $300 for a new spring assy and the horsehair pad. So I started watching ebay. I bought a very good used spring and a new horsehair from "ideal25" for around $150.00. I am very happy with it, it was a barely used spring, and he sold me a brand new OEM horshair pad. The old spring had multiple broken wires, and the horsehair had disintegrated. What a difference! Like driving a new car.

Go to ebay and look up ideal25. He usually has a pad or a spring for auction. I'm sure if you contact him via ebay, he'll sell you a set.

Another option:
Some have said to take the seat apart, then cut up those pool foam toys that are like sticks (I have no idea what they are called. They are very soft and kids like to hit each other with them or float on them. Sold at walMart). The pieces are then wedged into the springs. This is supposed to provide very good support. For a few bucks and a couple of hours of your time, you might be able to do a cheap repair.

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport