Got my set of rear shoulder belts yesterday and so I decided to put them in. As its been mentioned before, its simple and all the mounting holes are already there. The only down side is that the retracting part of the belts are exposed.

 

Nice! It looks like you re-used the front ones and put them in the back. Another option instead of finding new or used ones is to use universal ones.
https://www.gotbelts.com/retractable...cts/81/view/78

They offer plenty of different styles and colors.

 

Or use use front ones for a pick-up. Those have covers.

 

either way, i'm happy now that I have them even if they're exposed a little bit. I just really wanted to be able to have my son ride with me and be able to sit him in his booster chair.