My intention here is to help future people who do a Google search in this frustrating issue.

Firstly get a NEW UNUSED BRAND NEW IN THE BOX ISM/DSM UNIT.
I tried with three used and VIRGINIZED ism dsm units. Sellers are either liars or dont know what they are talking about. Never worked.

If I had just purchased the New ism dsm unit from the start then this job would have been completed in around 1 hour.
Mercedes dealers refused to sell me the unit. Tried several in sputher California. thankfully a seller called EXO PARTS out of Ontarip Canada had the ISM DSM in stock. I was u happy about it costing almost double what it costs from Mercedes, but I understand people are running a business. The seller is very professional and shipped it same day. I opted for next day air and I received it around 30 hours after paying for it.
BRAND NEW IN THE MERCEDES BOX.
do not buy the fake Ebay ism dsm units made by ripoff artists! This job is already costing you a lot of money, dont put yourself in a situation where you have to repeat this repair.

the ism dsm is located in the driver side. It is on the side of the transmission. Held on by four t30 bolts. You MUST REPLACE THESE BOLTS only 1.50 each.
I did not remove any parts to be able to remove the ism dsm unit. My hands are not small but I'm not large either. You need to use your flexibility and long needle nose pliers to hold one particular bolt in place while you use your finger tips to screw it in.
You will squeeze your hands in there for 2 top bolts. Lower teo bolts are more visible and accessible.

you will need a short or shallow t30 tool. It needs to be extended or long, the handle, so that it can reach the top bolts. There are extended 8mm spanner wrench ratcheting that you the insert these t30 bits that are 8mm in size, you get them from amazon.

You need a Mercedes computer, not just a scanner. You need scn coding. This is not available from the basic star diagnostics computer setup. If you do not want to buy the AUTEL machine from Amazon because its 2700 dollars, then you need to contact a Mercedes electronics guy, such as those who offer replacement keys, because their machine can do SCN coding. Generally the cost seems to be $300 for this service.

You need to firstly go into the ISM control unit and into control init adaptations, detach transport thing, then calibrate. Then go into programming and programme the ism.

Note that your car will likely be in Park. You need to use pliers and turn the protruding part on th side of the transmission towards the REAR of the car to put it into the Drive position. If you just want to get the car into NEUTRAL TO ALLOW IT TO BE pulled onto a tow truck, then you can also just turn the thing to the position that is one before last. Basically your transmission is in park, turn it one position to the rear of the car, that is Reverse, next botch is Neutral, next notch is Drive.

if you find that you cannot get under the car to remove the ISM and put I in Neutral, then ask the tow truck to bring skates or rollers to allow the 4 wheels to not be dragged along the ground because this can DAMAGE THE TRANSMISSION. if they dont have skates you will use several bottles of DISH SOAP and gallons and gallons of water. To reduce the friction caused by the tires. I had to do this and I gave the tow guy 60 dollars to slowly pull the car and let it down. He was not going to be patient if I did not promise him money. The drivers a re tired and this makes them happier to work calmly.

I have one brand new MB ISM if someone needs.

do you still the ISM?---I know it is old Blog

A note on removing the ISM/DSM from an X164 GL, and maybe other models, you do NOT have to remove the drive/propeller shaft to the front differential! Once the 4- T30 screws are removed (which on the X164, are metal, and can be reused), the box can be manipulated forward, and EASILY removed. The trick is removing the top two T30 screws, and that can be done with a short T30 bit, that you tap/press into a 6mm tubing/flare nut wrench; then zip-tie that tubing wrench to your longest and thinnest, combination wrench, using the "box" of each wrench as a zip-tie loop through point. In my case I have a thin double-ended 9" long, E-Torx wrench, I have used on my C280, to remove some engine bolts where a socket and driver could not fit. Strapping the two wrenches together the way I did, allowed for adjustable length by "telescoping", as the two wrenches were able to slide on each other with the zip-ties staying in place because they were threaded through each wrench's box loops. Also, my screws turned very easily, by just single finger touch, once the screw was loosened by less than 1 revolution.