i was reading up on layouts and circuit designs of car equipment, and found this site. if you're good at soldering, or at least have the jist of it, you can purchase DIY kits, or otherwise purchase pre-made kits.
this first link is talking about how to design a frequency switch for use like a shift light or something: Frequency Switch
at the bottom of the article there's a link to two pages. one for kits and one for DIY kits. some pretty cool stuff you can make for cheap, like an AFR guage for $11.50 DIY Kits