Hi smallchange,
I purchased the MB CDs off a vendor on EBay, just so I know if there is some job I can do, or if I really should get someone else with the specialized knowledge and special tools to do the work. Years ago, I spoke 'fluent Volvo' at several V. dealerships in & around Boston. I've little experience w Mercedes-benz, save the year plus I've had my 560.
For repair work (quite a bit last driving season) I'm pleased with the results I've gotten at Smith Motor Sales up in Haverhill, off Rte 495. That is a real hike from Boston (93N > 495N > Haverhill) for you.
There is an MB dealership much closer to me in Natick, MA (off 9 West) where I had a somewhat expensive & less than acceptable service experience. Having spent many years in high line European auto service: it is hard to trust someone else to do the service work correctly and not take shortcuts at my expense. At Smith Motors I feel I have an excellent working relationship with both the Parts & Service departments.
I take it, you're not going back to Chambers.... ?
For MB service closer to you, I'd ck with Cars.com > CarTalk section. I'd look at their '
MechanXfiles' section. Look for an MB shop, probably across the river in Cambridge or Somerville. See if you can find a tech with grey hair. Yup, I said that. He'll be more likely to have had much more W126 experience than a younger person. All the shops listed will have ratings (& have been recommended) by the shops' own customers. [

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You will spend some $$ getting that work done. You won't regret it, if your experience was anything like mine. It'll tighten things up quite a bit.
Couldn't hurt, Good Luck, MBL