Yes, body colored are original, and shiny metal is replaced.

1. They provide resistance when opening the door (so it doesn't flap loosely on the hinges), hold the door open and prevent it from closing on you (helpful when parking on an incline) and also prevent the door from opening too far.

2. They cost around $19 each and are available at either Fastlane or Partsshop

3. I'm not sure how often they go. My 1987 still has all four original checkstraps which operate very smoothly.

3. By "why do they go" I'm assuming you mean why they go bad and make loud cracking noises. Either the grease is gone or the bearings inside are worn out. Occasionally I've seen broken straps (the metal band broke in half) but I'm not sure what caused that.

4. You can lubricate the straps with some grease, which might stop it from making noise, but if the bearings are worn out, you'll need to replace it. The new style straps have a black plastic cover which helps keep the grease inside.

5. Replacement is easy. Remove the door panel, remove three bolts, and tap the pin out.