I'm doing a W116 so yours may be different.
I can get to almost the entire coil through the boot between the blower and center section here:
I can get to the rest through a rubber boot/plug on the other side behind the instruments:
It's a tedious work but I'm getting it a lot cleaner than I thought would be possible.
In hindsight I think the best way to do it would be to rig up a vacuum wand and suck all the dry debris out first, then use a compressed air wand to dislodge any loose debris and then flood it with water. Then use some commercial cleaner to get the fins shiny!