The "grunge" may well be because of the PEARL in the white trim code deteriorating and agig. NO cleaner will remove that that I've found.
One of the things Chrysler did back then to make their interiors "sparkle" next to a similar Ford or GM interior was to put a pearl tint on the vinyl and metallic threads in the cloth for the seats. Many missed that, until you put the color swmatches of material next to each other.
The best comparison is between the 1973 (pearlized) interior vinyls and the 1974 (non-pearlized) vinyls. The added (1973 and prior) pearl makes the 1974-era vinyl look very plain, by comparison, unfortunately. I was not aware of this until an upholstery shop owner showed it to me in about 1973.
In getting my '67 CE23 cleaned up after I bought in (1981), there was/is some areas near the corners of the front bucket seat backs that looks grungy, like somebody with greasy hands had been there, but no cleaner I found could remove it. That's when I determined it was from the pearl tint deteriorating/fading . . . which is why no cleaner would remove it.
ALL vinyls, inside and out.
Just my experiences,
CBODY67