i cleaned my evap in my w123 and my dad's old w123 with foaming coil clean from home depot. i dropped the blower and was able to completely clean it. i used my garden hose sprayer on different patterns to get all the crap out and kept my shop vac running to prevent the water from over flowing if the condensation drain were to clog. did the same on my w126. i would not keep the garden hose spraying for long periods as the water can't drain as quickly. also make sure your condensation drain isn't rotten as you will flood your car. if you stick a crevice attachment on your shop vac, you can get all the leaves, ect from the back corner but be very careful because you can easily damage the fins. as for the heater core, i wouldn't worry because the evaporator has probably filtered everything i stuck my phone in there to take pics and see if i missed any spots too! these pics were from the w126...

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

Thank you, this is quite helpful. I'll give it a try.

Thanks much,

Packman

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83 240D - 4 speed manual - Manilla Beige
189K miles, Tachometer mod, cool wooden shift knob from PeachParts, CocoMats, Original factory paint, manual windows, manual sunroof. Starting to add AudioWrap to this car too!