Thanks again for all of your suggestions. I just joined the Forum yesterday and still don't know what I am doing. I did post a thread on the W220 Forum but have gotten no replies as yet. I also took your suggestion and updated my profile, which up until then, I didn't know there was a place for that. Once I get this car in good order, I'll check out the R129 forum. I'm in Florida for a couple of weeks, but when I get back up North, I'll check with an indie shop.
My trunk actually opens fine....with the key, with the remote and with the button on the door. There is only resistance when closing it. At least I don't have to worry about it falling on my head. Maybe it has something to do with the springs. Could they be gas springs? Could they only have a problem in one direction?
I will try to start with the sensor for the ambient temp fix. I can see the sensor in the bumper grill by the driver side headlight, but I don't know how to get at it. On my SA, it was easy....the grill piece just popped out, and the sensor was attached to the grill, and the wiring just plugged into the back of the grill. I don't think the grill piece on the S500 pops out.
On the headrests, I'm hoping that someone can tell me how to release the rear headrests without the switch working. I would just make sure they stayed down in the future.
With the parking brake, the plastic handle is in its track on the lower part of the dash, but I can pull it all the way out since it's not connected to anything. It doesn't appear to have anything broken on it. With a flashlight, while the handle is out, I can see what looks like the cable parts that should attach to the handle. However, I'm afraid to go in there with pliers to try to pull it out in order to try to attach....I'm afraid I'll pull too hard and snap the cable.
I expect your parking brake release cable has either stretched or the adjuster has worked its way loose. Intuition (not knowledge) says the adjuster will be at the rear wheels. I figure you could loosen the cable adjuster, refasten the cable to the release in the cabin, then tighten the adjuster again to take the slack out. Sounds like it has come loose in the cabin because it was slack. If you aren't handy, I'd suggest you write a check for somebody else to do it. If nothing else, it will save you some anxiety about whether you are doing the right thing.
The headrests can probably be released from somewhere around them. I don't know if you'd get at the mechanism from the top or the trunk. Either way, it will likely require the removal of some trim pieces, so it is likely another case of writing a check for somebody who knows what they are doing if you don't.
That trunk thing is still confusing to me. It sounds like there is some binding on the downward movement if you don't have power close, but I don't know enough about the way that hinge system is constructed to venture a guess about where or what it would be. This is definitely a W220 forum question.
I'm with you on the R129 sensor--mine was missing when I bought my car, but it was a simply DIY fix. I have never had any problems with my W220, so I suggest (again) that a tech who knows the car will do it more efficiently than you could.
I keep going back to paying someone, and I don't mean to insult your handiness by doing so. I'm a pretty fair shade-tree, and I have a big roll-around tool box but I usually pay to have anything done on my two MRZ other than routine filter changes or things that are little more than a screwdriver or pair of pliers. They are just too complex and subject to all manner of catastrophe if I muddle around with them too much. If you are handy and confident you could probably do most of these repairs themselves. I don't have a lift or ramps, and wallowing around on my garage floor isn't something I'm inclined to do much any more.