I would use a very mild soap. Like a cap full of Woolite soap diluted in bucket.
I have had amazing results with just hot soapy water on all parts of the car...form top to rugs, to even leather (damp microfibre towel, wrung out often).
I sponge it on first with one soapy bucket, then wipe w/wet towel w/ clean hot water in another bucket; then vacuum water with shop-vac, and spot-dry excess with dry towel.
Each time, I had 2-3 buckets of black water on what looked like a clean surface.
The soot and tar from exhaust/cigar/cigarette smoke really turns everything stiff and brittle....near steaming hot water dissolves it.
See my tip on leather conditioner....Byrons Hyde Food. My 8yr leather GAHH seat kit / dash / door cards went from shiny / stiff to supple, matte/satin and much richer cognac color like a new Bentley / Rolls!
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