So I found Cisco's on 9851 8th Street, Suite B
Rancho Cucamonga, CA 91730. Cisco is the owner and is easy to work and talk to.
I can back this up. Cisco does great work at a fair price, and he's been servicing my W210 E55 since I moved to SoCal in mid-2013. Honest, reliable, fair price... and he does warranty his work. That said, he's awful with communication, and within the last year, he's been unable to perform the relatively quick repairs that I'd come to expect. His shop has grown a bit too big for him - too much work, and not enough time. The last two times I had to service there (my W210 E55 and my mother's W211 E350), the work took up to 2-3 days longer than originally estimated... which can be a problem if you don't have a spare car.
Since I got my W211 E55, I've been servicing with Upland Mercedes Repair, at 923-H N Central Ave
Upland, CA 91786. It's run by Assad, and is a smaller shop with a more personal touch. Excellent communication, and very fast service - at extremely reasonable labor rates (i think it's $90 an hour?)...
You can't go wrong with either shop, but allow more time for Cisco to do the work. Since I live in Claremont, taking the car 10 minutes down the road to Upland has been easier and Assad is very competent...