This is a shot of the "old" style spring that originally came in the early 80's cars. This one came out of my 300SD. You can replace just this spring and it is usually the culprit. If it is a car with power seats plan on taking 4 to 6 hours to change it out. If they are not power seats should be doable in about 2 to 3 hours. It is a good diy'er type of job
. More pia than hard. I work pretty slow so my times may be longer than someone that does it all the time.
Oh yea, don't get one from a junkyard or a "used" one off of ebay. You will more than likely be getting one like what is shown. There has been a design change which improves it alot.