Instructables has a very clever project for 3D printers. Replacing your old car parts with a newly printed one. Obviously this won’t yet work for ALL car parts, but it will certainly come in handy for some…
From the site:
Here is a simple instructable to 3d print new replacement parts for virtually an object with plastic parts that are hard to obtain or no longer available in this case a 1950′s classic car water pump component. Be mindful of your intended parts use, 3d printing may not be suitable for high stress, heat and generally extreme functional uses. You may need to make a two part silicone mold and reproduce the print in a resin of suitable properties.
Collect as much information you can. In this case the original broken part that needed replacing was available broken but still in good enough condition to take measurements.
I can highly recommend getting a set of Digital calipers, normally under $30 from ebay and amazon etc. measure all facets of the part twice before inserting the dimensions into your computer model. Currently I’m using Solidworks but feel free to use google sketch-up Rhino, 3ds max or any other 3D software you can get your hands on. Its easier to check twice now than it is to fix mistakes from incorrect measurements after the fact.