For my Coupe I bought a deep cheap bucket at the Dollar store and some soft bristled brushes and a fingernail brush plus a package of clothes pins .
I used Meguiars #40 plastic & leather cleaner / conditioner but I'm sure most any upholstery cleaner will do the truck . I'm now out of Meguiars and don't know who stocks it so I'll prolly try foaming upholstery cleaner from my F.L.P.S. next, I really want to do the Wagon's belts before another buckle gets broken .
I slid the seat back until the bucket lined up with the seat belt where it hands and unspooled the seat belt as far as it'd go then used the clothes pins (3) right where it comes out and let it fall into the bucket, sprayed the webbing liberally and worked it into the fabric with my hands .
I then left it to soak for an hour before coming back and spraying it and the brush with the cleaner then I used the brush and my hands down in the bucket to prevent spattering the seats or carpet with dirty soapy crud, to gently brush the belt, working from one end to the other .
This isn't hard but the belt keeps wanting to crease or flip over in your hands and it's very important to not skip any parts of it .
Then I wiped it with clean rags, rinsed out the bucket and filled it with warm water about 3/4 full and put the belt back in & worked the brush all over it again , the water turned dead black so after a while I used two fingers to wring the belt as I pulled it out, dumped the dirty water and re filled the bucket, I did this over and over until I got clear water with no soap, dirt, bubbles floating etc. in it .
Then I used more clean dry towels to dry the belt and looped it over the back rest and let it air dry ~ the belt's fabric is synthetic so it only took a few hours to dry .
The Sedan , I removed the entire belt and retractor for cleaning (picture was of the belt in my kitchen sink) , I can detail that too if you'd like, it's not difficult .
I hope this helps .
-Nate