I know this is stupid but after I replaced my carpets, my interior of my '71 was very clean except for those luggage straps.  The sun faded them and/or the years stained them a dull grey brown. Cleaning them didn't help. I wanted to get some fabric dye but I heard the thickness can change the texture of the fabric.  I know the straps are nylon but I am painfully picky. I wasn't willing to take any chance.  I got gutsy and bought a bottle of liquid "Kiwi scuff cover" shoe polish at wal mart. The stuff is watery thin so you cant over apply or under apply.  The straps look new again but not artificially black.

Also, the previous body shop didn't realize those z emblems on the b pillers were vents.  I got a little 905 red overspray on my black diamond vinyl Kiwi scuff cover worked for that too.  

Edited December 4, 20168 yr by 87mj

RIT fabric dye is what I used to get my faded straps back to a nice black - no change to the thickness or texture of the material.  Soak the straps in a bucket with the dye and water for about 15 min., stir a few times, let dry, reinstall.

I did the same as Jim with no thickness or texture change. RIT has been around a very long time.  

Glad you way worked also. It's always good to have an alternate method.