I've been neglecting my poor leather interior lately(about the last 50,000 miles), and finally got around to giving it a proper cleaning.
I'm using GC Leather Cleaner/Conditioner

and a horsehair toothbrush that I've had for a few years
and a WHOLE LOT of paper towels
I start by pulling a small amount of cleaner onto the bristles of the brush, then work it into the leather using small circular motions. This allows the dirt and grime on the seats to emulsify with the cleaner and draws it off of the surface. I then wipe the cleaner off the leather using a folded paper towel, and use the same dirty towel to clean the toothbrush which has gotten dirty through the scrubbing action.
You can see in this picture that it's a VERY effective product and method:

The area at top left and the uppermost interior panel of the base have not been cleaned yet in the above picture.
Due to educational commitments, finishing the seats up will have to wait until tomorrow afternoon, so I'll just leave you with the above as a teaser
It's a time consuming method, but highly effective in cleaning neglected leather. I'm sure that a larger brush would expedite the process without sacrificing effectiveness for people with time constraints.
I'm using GC Leather Cleaner/Conditioner

and a horsehair toothbrush that I've had for a few years
and a WHOLE LOT of paper towels
I start by pulling a small amount of cleaner onto the bristles of the brush, then work it into the leather using small circular motions. This allows the dirt and grime on the seats to emulsify with the cleaner and draws it off of the surface. I then wipe the cleaner off the leather using a folded paper towel, and use the same dirty towel to clean the toothbrush which has gotten dirty through the scrubbing action.
You can see in this picture that it's a VERY effective product and method:

The area at top left and the uppermost interior panel of the base have not been cleaned yet in the above picture.
Due to educational commitments, finishing the seats up will have to wait until tomorrow afternoon, so I'll just leave you with the above as a teaser

It's a time consuming method, but highly effective in cleaning neglected leather. I'm sure that a larger brush would expedite the process without sacrificing effectiveness for people with time constraints.
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