I've seen what steam cleaning can do, like peel off paint and ruin wiring harnesses.
I only use WD-40 and lots of it! Use an old toothbrush and work on the areas then wipe the grunge off as you go, using the blue papertowels they sell for automotive use or old socks are great for this.
I sometimes follow the WD-40 treatment with a shot of Simple Green and wipe it off with a towel.

For dirty aluminum parts, topside fenderwells and wheels I use the Orange stuff (go-jo?) hand cleaner that has a little grit in it, you can do wonders with that stuff!

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body. '87 300D w/ Sportline suspension. No more '79 300TD w/ 617.952 engine at 367,750, it was my annual driver to Texas and I sure don't miss it poor A/C heater system!