I have the 14in VW alloy wheels on my 1987. A few weeks ago I had a rear tire changed and when the shop did it they omitted to reinstall the plastic center cap. Since I could not figure out how to re-install it, I just left it off and did not think twice about it. When filling up for gas, I now noticed that that alloy wheel (and none of the other three) is covered in blackish gray stain. I'm assuming this is brake dust? Does the center cap really prevent this or is it just a concidence? And just how to I get this wheel looking shiny aluminum again. I tried using toothpaste and it had little effect. How do I reinstall the center cap? Thanks.

-David (1987 Wolfsburg GL, Tempe AZ)